Managing read tags in a transactional memory

ABSTRACT

Managing cache evictions during transactional execution of a process. Based on initiating transactional execution of a memory data accessing instruction, memory data is fetched from a memory location, the memory data to be loaded as a new line into a cache entry of the cache. Based on determining that a threshold number of cache entries have been marked as read-set cache lines, determining whether a cache entry that is a read-set cache line can be replaced by identifying a cache entry that is a read-set cache line for the transaction that contains memory data from a memory address within a predetermined non-conflict address range. Then invalidating the identified cache entry of the transaction. Then loading the fetched memory data into the identified cache entry, and then marking the identified cache entry as a read-set cache line of the transaction.

BACKGROUND

The present disclosure relates generally to the field of cachecoherency, and more particularly to selectively evicting read-set cachelines within a transactional memory environment.

The number of central processing unit (CPU) cores on a chip and thenumber of CPU cores connected to a shared memory continues to growsignificantly to support growing workload capacity demand. Theincreasing number of CPUs cooperating to process the same workloads putsa significant burden on software scalability; for example, shared queuesor data-structures protected by traditional semaphores become hot spotsand lead to sub-linear n-way scaling curves. Traditionally this has beencountered by implementing finer-grained locking in software, and withlower latency/higher bandwidth interconnects in hardware. Implementingfine-grained locking to improve software scalability can be verycomplicated and error-prone, and at today's CPU frequencies, thelatencies of hardware interconnects are limited by the physicaldimension of the chips and systems, and by the speed of light.

Implementations of hardware Transactional Memory (HTM, or in thisdiscussion, simply TM) have been introduced, wherein a group ofinstructions—called a transaction—operate in an atomic manner on a datastructure in memory, as viewed by other central processing units (CPUs)and the I/O subsystem (atomic operation is also known as “blockconcurrent” or “serialized” in other literature). The transactionexecutes optimistically without obtaining a lock, but may need to abortand retry the transaction execution if an operation, of the executingtransaction, on a memory location conflicts with another operation onthe same memory location. Previously, software transactional memoryimplementations have been proposed to support software TransactionalMemory (TM). However, hardware TM can provide improved performanceaspects and ease of use over software TM.

U.S. Pat. No. 7,269,694 titled “Selectively Monitoring Loads to SupportTransactional Program Execution,” filed Aug. 8, 2003, by Tremblay et al.(“Tremblay 2003”), and incorporated by reference herein in its entiretyteaches a system that selectively monitors load instructions to supporttransactional execution of a process, wherein changes made during thetransactional execution are not committed to the architectural state ofa processor until the transactional execution successfully completes.Upon encountering a load instruction during transactional execution of ablock of instructions, the system determines whether the loadinstruction is a monitored load instruction or an unmonitored loadinstruction. If the load instruction is a monitored load instruction,the system performs the load operation, and load-marks a cache lineassociated with the load instruction to facilitate subsequent detectionof an interfering data access to the cache line from another process. Ifthe load instruction is an unmonitored load instruction, the systemperforms the load operation without load-marking the cache line.

U.S. Pat. No. 8,209,499 titled “Method of Read-Set and Write-SetManagement by Distinguishing Between Shared and Non-Shared MemoryRegions,” filed Jan. 15, 2010, by Chou (“Chou 2010”), and incorporatedby reference herein in its entirety, teaches a method of read-set andwrite-set management that distinguishes between shared and non-sharedmemory regions. A shared memory region, used by a transactional memoryapplication, which may be shared by one or more concurrent transactionsis identified. A non-shared memory region, used by the transactionalmemory application, which is not shared by the one or more concurrenttransactions is identified. A subset of a read-set and a write-set thataccess the shared memory region is checked for conflicts with the one ormore concurrent transactions at a first granularity. A subset of theread-set and the write-set that access the non-shared memory region ischecked for conflicts with the one or more concurrent transactions at asecond granularity. The first granularity is finer than the secondgranularity.

SUMMARY

Embodiments of the disclosure describe a method, computer programproduct, and system for managing cache line evictions duringtransactional execution of a process. Based on initiating transactionalexecution of a memory data accessing instruction, memory data is fetchedfrom a memory location, at a memory address specified by theinstruction, the memory data to be loaded as a new line into a cacheentry of the cache. Based on determining that a threshold number ofcache entries for the transaction in the cache have been marked asread-set cache lines, determining whether a cache entry that is aread-set cache line can be replaced, by identifying, in the cache, acache entry that is a read-set cache line for the transaction, theidentified cache entry containing memory data from a memory addresswithin a predetermined non-conflict address range. Then invalidating theidentified cache entry of the transaction. Then loading the fetchedmemory data into the identified cache entry, and then marking theidentified cache entry as a read-set cache line of the transaction.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

One or more aspects of the present disclosure are particularly pointedout and distinctly claimed as examples in the claims at the conclusionof the specification. The foregoing and other objects, features, andadvantages of the disclosure are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 depicts a functional block diagram of a multicore transactionalmemory environment, in accordance with embodiments of the disclosure;

FIG. 2 depicts a functional block diagram of a CPU core of the multicoretransactional memory environment of FIG. 1, in accordance withembodiments of the disclosure;

FIG. 3 depicts a functional block diagram of components of a CPU, inaccordance with embodiments of the disclosure;

FIG. 4 shows an embodiment of a multicore TM environment, in accordancewith an embodiment of the present disclosure;

FIG. 5 is a flowchart depicting operational steps for performing anaspect of cache line evictions during transactional execution of aprocess, in accordance with an embodiment of the disclosure; and

FIG. 6 is a flowchart depicting operational steps for performing asecond aspect of cache line evictions during transactional execution ofa process, in accordance with an embodiment of the disclosure.

FIGS. 7A and 7B are a flowchart depicting operational steps forperforming another aspect of cache line evictions during transactionalexecution of a process, in accordance with an embodiment of thedisclosure.

FIG. 8 depicts a block diagram of components of an embodiment of acomputing device that may include the multicore transactional memoryenvironment of FIG. 1, the CPU core of a multicore transactional memoryenvironment of FIG. 2, and/or the components of the CPU of FIG. 3, inaccordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

Historically, a computer system or processor had only a single processor(aka processing unit or central processing unit). The processor includedan instruction processing unit (IPU), a branch unit, a memory controlunit and the like. Such processors were capable of executing a singlethread of a program at a time. Operating systems were developed thatcould time-share a processor by dispatching a program to be executed onthe processor for a period of time, and then dispatching another programto be executed on the processor for another period of time. Astechnology evolved, memory subsystem caches were often added to theprocessor as well as complex dynamic address translation includingtranslation lookaside buffers (TLBs). The IPU itself was often referredto as a processor. As technology continued to evolve, an entireprocessor, could be packaged in a single semiconductor chip or die, sucha processor was referred to as a microprocessor. Then processors weredeveloped that incorporated multiple IPUs, such processors were oftenreferred to as multi-processors. Each such processor of amulti-processor computer system (processor) may include individual orshared caches, memory interfaces, system bus, address translationmechanism and the like. Virtual machine and instruction set architecture(ISA) emulators added a layer of software to a processor, that providedthe virtual machine with multiple “virtual processors” (aka processors)by time-slice usage of a single IPU in a single hardware processor. Astechnology further evolved, multi-threaded processors were developed,enabling a single hardware processor having a single multi-thread IPU toprovide a capability of simultaneously executing threads of differentprograms, thus each thread of a multi-threaded processor appeared to theoperating system as a processor. As technology further evolved, it waspossible to put multiple processors (each having an IPU) on a singlesemiconductor chip or die. These processors were referred to processorcores or just cores. Thus the terms such as processor, centralprocessing unit, processing unit, microprocessor, core, processor core,processor thread, and thread, for example, are often usedinterchangeably. Aspects of embodiments herein may be practiced by anyor all processors including those shown supra, without departing fromthe teachings herein. Wherein the term “thread” or “processor thread” isused herein, it is expected that particular advantage of the embodimentmay be had in a processor thread implementation.

Transaction Execution in Intel® Based Embodiments

In “Intel® Architecture Instruction Set Extensions ProgrammingReference” 319433-012A, February 2012, incorporated herein by referencein its entirety, Chapter 8 teaches, in part, that multithreadedapplications may take advantage of increasing numbers of CPU cores toachieve higher performance. However, the writing of multi-threadedapplications requires programmers to understand and take into accountdata sharing among the multiple threads. Access to shared data typicallyrequires synchronization mechanisms. These synchronization mechanismsare used to ensure that multiple threads update shared data byserializing operations that are applied to the shared data, oftenthrough the use of a critical section that is protected by a lock. Sinceserialization limits concurrency, programmers try to limit the overheaddue to synchronization.

Intel® Transactional Synchronization Extensions (Intel® TSX) allow aprocessor to dynamically determine whether threads need to be serializedthrough lock-protected critical sections, and to perform thatserialization only when required. This allows the processor to exposeand exploit concurrency that is hidden in an application because ofdynamically unnecessary synchronization.

With Intel TSX, programmer-specified code regions (also referred to as“transactional regions” or just “transactions”) are executedtransactionally. If the transactional execution completes successfully,then all memory operations performed within the transactional regionwill appear to have occurred instantaneously when viewed from otherprocessors. A processor makes the memory operations of the executedtransaction, performed within the transactional region, visible to otherprocessors only when a successful commit occurs, i.e., when thetransaction successfully completes execution. This process is oftenreferred to as an atomic commit.

Intel TSX provides two software interfaces to specify regions of codefor transactional execution. Hardware Lock Elision (HLE) is a legacycompatible instruction set extension (comprising the XACQUIRE andXRELEASE prefixes) to specify transactional regions. RestrictedTransactional Memory (RTM) is a new instruction set interface(comprising the XBEGIN, XEND, and XABORT instructions) for programmersto define transactional regions in a more flexible manner than thatpossible with HLE. HLE is for programmers who prefer the backwardcompatibility of the conventional mutual exclusion programming model andwould like to run HLE-enabled software on legacy hardware but would alsolike to take advantage of the new lock elision capabilities on hardwarewith HLE support. RTM is for programmers who prefer a flexible interfaceto the transactional execution hardware. In addition, Intel TSX alsoprovides an XTEST instruction. This instruction allows software to querywhether the logical processor is transactionally executing in atransactional region identified by either HLE or RTM.

Since a successful transactional execution ensures an atomic commit, theprocessor executes the code region optimistically without explicitsynchronization. If synchronization was unnecessary for that specificexecution, execution can commit without any cross-thread serialization.If the processor cannot commit atomically, then the optimistic executionfails. When this happens, the processor will roll back the execution, aprocess referred to as a transactional abort. On a transactional abort,the processor will discard all updates performed in the memory regionused by the transaction, restore architectural state to appear as if theoptimistic execution never occurred, and resume executionnon-transactionally.

A processor can perform a transactional abort for numerous reasons. Aprimary reason to abort a transaction is due to conflicting memoryaccesses between the transactionally executing logical processor andanother logical processor. Such conflicting memory accesses may preventa successful transactional execution. Memory addresses read from withina transactional region constitute the read-set of the transactionalregion and addresses written to within the transactional regionconstitute the write-set of the transactional region. Intel TSXmaintains the read- and write-sets at the granularity of a cache line. Aconflicting memory access occurs if another logical processor eitherreads a location that is part of the transactional region's write-set orwrites a location that is a part of either the read- or write-set of thetransactional region. A conflicting access typically means thatserialization is required for this code region. Since Intel TSX detectsdata conflicts at the granularity of a cache line, unrelated datalocations placed in the same cache line will be detected as conflictsthat result in transactional aborts. Transactional aborts may also occurdue to limited transactional resources. For example, the amount of dataaccessed in the region may exceed an implementation-specific capacity.Additionally, some instructions and system events may causetransactional aborts. Frequent transactional aborts result in wastedcycles and increased inefficiency.

Hardware Lock Elision

Hardware Lock Elision (HLE) provides a legacy compatible instruction setinterface for programmers to use transactional execution. HLE providestwo new instruction prefix hints: XACQUIRE and XRELEASE.

With HLE, a programmer adds the XACQUIRE prefix to the front of theinstruction that is used to acquire the lock that is protecting thecritical section. The processor treats the prefix as a hint to elide thewrite associated with the lock acquire operation. Even though the lockacquire has an associated write operation to the lock, the processordoes not add the address of the lock to the transactional region'swrite-set nor does it issue any write requests to the lock. Instead, theaddress of the lock is added to the read-set. The logical processorenters transactional execution. If the lock was available before theXACQUIRE prefixed instruction, then all other processors will continueto see the lock as available afterwards. Since the transactionallyexecuting logical processor neither added the address of the lock to itswrite-set nor performed externally visible write operations to the lock,other logical processors can read the lock without causing a dataconflict. This allows other logical processors to also enter andconcurrently execute the critical section protected by the lock. Theprocessor automatically detects any data conflicts that occur during thetransactional execution and will perform a transactional abort ifnecessary.

Even though the eliding processor did not perform any external writeoperations to the lock, the hardware ensures program order of operationson the lock. If the eliding processor itself reads the value of the lockin the critical section, it will appear as if the processor had acquiredthe lock, i.e. the read will return the non-elided value. This behaviorallows an HLE execution to be functionally equivalent to an executionwithout the HLE prefixes.

An XRELEASE prefix can be added in front of an instruction that is usedto release the lock protecting a critical section. Releasing the lockinvolves a write to the lock. If the instruction is to restore the valueof the lock to the value the lock had prior to the XACQUIRE prefixedlock acquire operation on the same lock, then the processor elides theexternal write request associated with the release of the lock and doesnot add the address of the lock to the write-set. The processor thenattempts to commit the transactional execution.

With HLE, if multiple threads execute critical sections protected by thesame lock but they do not perform any conflicting operations on eachother's data, then the threads can execute concurrently and withoutserialization. Even though the software uses lock acquisition operationson a common lock, the hardware recognizes this, elides the lock, andexecutes the critical sections on the two threads without requiring anycommunication through the lock—if such communication was dynamicallyunnecessary.

If the processor is unable to execute the region transactionally, thenthe processor will execute the region non-transactionally and withoutelision. HLE enabled software has the same forward progress guaranteesas the underlying non-HLE lock-based execution. For successful HLEexecution, the lock and the critical section code must follow certainguidelines. These guidelines only affect performance; and failure tofollow these guidelines will not result in a functional failure.Hardware without HLE support will ignore the XACQUIRE and XRELEASEprefix hints and will not perform any elision since these prefixescorrespond to the REPNE/REPE IA-32 prefixes which are ignored on theinstructions where XACQUIRE and XRELEASE are valid. Importantly, HLE iscompatible with the existing lock-based programming model. Improper useof hints will not cause functional bugs though it may expose latent bugsalready in the code.

Restricted Transactional Memory (RTM) provides a flexible softwareinterface for transactional execution. RTM provides three newinstructions—XBEGIN, XEND, and XABORT—for programmers to start, commit,and abort a transactional execution.

The programmer uses the XBEGIN instruction to specify the start of atransactional code region and the XEND instruction to specify the end ofthe transactional code region. If the RTM region could not besuccessfully executed transactionally, then the XBEGIN instruction takesan operand that provides a relative offset to the fallback instructionaddress.

A processor may abort RTM transactional execution for many reasons. Inmany instances, the hardware automatically detects transactional abortconditions and restarts execution from the fallback instruction addresswith the architectural state corresponding to that present at the startof the XBEGIN instruction and the EAX register updated to describe theabort status.

The XABORT instruction allows programmers to abort the execution of anRTM region explicitly. The XABORT instruction takes an 8-bit immediateargument that is loaded into the EAX register and will thus be availableto software following an RTM abort. RTM instructions do not have anydata memory location associated with them. While the hardware providesno guarantees as to whether an RTM region will ever successfully committransactionally, most transactions that follow the recommendedguidelines are expected to successfully commit transactionally. However,programmers must always provide an alternative code sequence in thefallback path to guarantee forward progress. This may be as simple asacquiring a lock and executing the specified code regionnon-transactionally. Further, a transaction that always aborts on agiven implementation may complete transactionally on a futureimplementation. Therefore, programmers must ensure the code paths forthe transactional region and the alternative code sequence arefunctionally tested.

Detection of HLE Support

A processor supports HLE execution if CPUID.07H.EBX.HLE [bit 4]=1.However, an application can use the HLE prefixes (XACQUIRE and XRELEASE)without checking whether the processor supports HLE. Processors withoutHLE support ignore these prefixes and will execute the code withoutentering transactional execution.

Detection of RTM Support

A processor supports RTM execution if CPUID.07H.EBX.RTM [bit 11]=1. Anapplication must check if the processor supports RTM before it uses theRTM instructions (XBEGIN, XEND, XABORT). These instructions willgenerate a #UD exception when used on a processor that does not supportRTM.

Detection of XTEST Instruction

A processor supports the XTEST instruction if it supports either HLE orRTM. An application must check either of these feature flags beforeusing the XTEST instruction. This instruction will generate a #UDexception when used on a processor that does not support either HLE orRTM.

Querying Transactional Execution Status

The XTEST instruction can be used to determine the transactional statusof a transactional region specified by HLE or RTM. Note, while the HLEprefixes are ignored on processors that do not support HLE, the XTESTinstruction will generate a #UD exception when used on processors thatdo not support either HLE or RTM.

Requirements for HLE Locks

For HLE execution to successfully commit transactionally, the lock mustsatisfy certain properties and access to the lock must follow certainguidelines.

An XRELEASE prefixed instruction must restore the value of the elidedlock to the value it had before the lock acquisition. This allowshardware to safely elide locks by not adding them to the write-set. Thedata size and data address of the lock release (XRELEASE prefixed)instruction must match that of the lock acquire (XACQUIRE prefixed) andthe lock must not cross a cache line boundary.

Software should not write to the elided lock inside a transactional HLEregion with any instruction other than an XRELEASE prefixed instruction,otherwise such a write may cause a transactional abort. In addition,recursive locks (where a thread acquires the same lock multiple timeswithout first releasing the lock) may also cause a transactional abort.Note that software can observe the result of the elided lock acquireinside the critical section. Such a read operation will return the valueof the write to the lock.

The processor automatically detects violations to these guidelines, andsafely transitions to a non-transactional execution without elision.Since Intel TSX detects conflicts at the granularity of a cache line,writes to data collocated on the same cache line as the elided lock maybe detected as data conflicts by other logical processors eliding thesame lock.

Transactional Nesting

Both HLE and RTM support nested transactional regions. However, atransactional abort restores state to the operation that startedtransactional execution: either the outermost XACQUIRE prefixed HLEeligible instruction or the outermost XBEGIN instruction. The processortreats all nested transactions as one transaction.

HLE Nesting and Elision

Programmers can nest HLE regions up to an implementation specific depthof MAX_HLE_NEST_COUNT. Each logical processor tracks the nesting countinternally but this count is not available to software. An XACQUIREprefixed HLE-eligible instruction increments the nesting count, and anXRELEASE prefixed HLE-eligible instruction decrements it. The logicalprocessor enters transactional execution when the nesting count goesfrom zero to one. The logical processor attempts to commit only when thenesting count becomes zero. A transactional abort may occur if thenesting count exceeds MAX_HLE_NEST_COUNT.

In addition to supporting nested HLE regions, the processor can alsoelide multiple nested locks. The processor tracks a lock for elisionbeginning with the XACQUIRE prefixed HLE eligible instruction for thatlock and ending with the XRELEASE prefixed HLE eligible instruction forthat same lock. The processor can, at any one time, track up to aMAX_HLE_ELIDED_LOCKS number of locks. For example, if the implementationsupports a MAX_HLE_ELIDED_LOCKS value of two and if the programmer neststhree HLE identified critical sections (by performing XACQUIRE prefixedHLE eligible instructions on three distinct locks without performing anintervening XRELEASE prefixed HLE eligible instruction on any one of thelocks), then the first two locks will be elided, but the third won't beelided (but will be added to the transaction's writeset). However, theexecution will still continue transactionally. Once an XRELEASE for oneof the two elided locks is encountered, a subsequent lock acquiredthrough the XACQUIRE prefixed HLE eligible instruction will be elided.

The processor attempts to commit the HLE execution when all elidedXACQUIRE and XRELEASE pairs have been matched, the nesting count goes tozero, and the locks have satisfied requirements. If execution cannotcommit atomically, then execution transitions to a non-transactionalexecution without elision as if the first instruction did not have anXACQUIRE prefix.

RTM Nesting

Programmers can nest RTM regions up to an implementation specificMAX_RTM_NEST_COUNT. The logical processor tracks the nesting countinternally but this count is not available to software. An XBEGINinstruction increments the nesting count, and an XEND instructiondecrements the nesting count. The logical processor attempts to commitonly if the nesting count becomes zero. A transactional abort occurs ifthe nesting count exceeds MAX_RTM_NEST_COUNT.

Nesting HLE and RTM

HLE and RTM provide two alternative software interfaces to a commontransactional execution capability. Transactional processing behavior isimplementation specific when HLE and RTM are nested together, e.g., HLEis inside RTM or RTM is inside HLE. However, in all cases, theimplementation will maintain HLE and RTM semantics. An implementationmay choose to ignore HLE hints when used inside RTM regions, and maycause a transactional abort when RTM instructions are used inside HLEregions. In the latter case, the transition from transactional tonon-transactional execution occurs seamlessly since the processor willre-execute the HLE region without actually doing elision, and thenexecute the RTM instructions.

Abort Status Definition

RTM uses the EAX register to communicate abort status to software.Following an RTM abort the EAX register has the following definition.

TABLE 1 RTM Abort Status Definition EAX Register Bit Position Meaning 0Set if abort caused by XABORT instruction 1 If set, the transaction maysucceed on retry, this bit is always clear if bit 0 is set 2 Set ifanother logical processor conflicted with a memory address that was partof the transaction that aborted 3 Set if an internal buffer overflowed 4Set if a debug breakpoint was hit 5 Set if an abort occurred duringexecution of a nested transaction 23:6 Reserved 31-24 XABORT argument(only valid if bit 0 set, otherwise reserved)

The EAX abort status for RTM only provides causes for aborts. It doesnot by itself encode whether an abort or commit occurred for the RTMregion. The value of EAX can be 0 following an RTM abort. For example, aCPUID instruction when used inside an RTM region causes a transactionalabort and may not satisfy the requirements for setting any of the EAXbits. This may result in an EAX value of 0.

RTM Memory Ordering

A successful RTM commit causes all memory operations in the RTM regionto appear to execute atomically. A successfully committed RTM regionconsisting of an XBEGIN followed by an XEND, even with no memoryoperations in the RTM region, has the same ordering semantics as a LOCKprefixed instruction.

The XBEGIN instruction does not have fencing semantics. However, if anRTM execution aborts, then all memory updates from within the RTM regionare discarded and are not made visible to any other logical processor.

RTM-Enabled Debugger Support

By default, any debug exception inside an RTM region will cause atransactional abort and will redirect control flow to the fallbackinstruction address with architectural state recovered and bit 4 in EAXset. However, to allow software debuggers to intercept execution ondebug exceptions, the RTM architecture provides additional capability.

If bit 11 of DR7 and bit 15 of the IA32_DEBUGCTL_MSR are both 1, any RTMabort due to a debug exception (#DB) or breakpoint exception (#BP)causes execution to roll back and restart from the XBEGIN instructioninstead of the fallback address. In this scenario, the EAX register willalso be restored back to the point of the XBEGIN instruction.

Programming Considerations

Typical programmer-identified regions are expected to transactionallyexecute and commit successfully. However, Intel TSX does not provide anysuch guarantee. A transactional execution may abort for many reasons. Totake full advantage of the transactional capabilities, programmersshould follow certain guidelines to increase the probability of theirtransactional execution committing successfully.

This section discusses various events that may cause transactionalaborts. The architecture ensures that updates performed within atransaction that subsequently aborts execution will never becomevisible. Only committed transactional executions initiate an update tothe architectural state. Transactional aborts never cause functionalfailures and only affect performance.

Instruction Based Considerations

Programmers can use any instruction safely inside a transaction (HLE orRTM) and can use transactions at any privilege level. However, someinstructions will always abort the transactional execution and causeexecution to seamlessly and safely transition to a non-transactionalpath.

Intel TSX allows for most common instructions to be used insidetransactions without causing aborts. The following operations inside atransaction do not typically cause an abort:

-   -   Operations on the instruction pointer register, general purpose        registers (GPRs) and the status flags (CF, OF, SF, PF, AF, and        ZF); and    -   Operations on XMM and YMM registers and the MXCSR register.

However, programmers must be careful when intermixing SSE and AVXoperations inside a transactional region. Intermixing SSE instructionsaccessing XMM registers and AVX instructions accessing YMM registers maycause transactions to abort. Programmers may use REP/REPNE prefixedstring operations inside transactions. However, long strings may causeaborts. Further, the use of CLD and STD instructions may cause aborts ifthey change the value of the DF flag. However, if DF is 1, the STDinstruction will not cause an abort. Similarly, if DF is 0, then the CLDinstruction will not cause an abort.

Instructions not enumerated here as causing abort when used inside atransaction will typically not cause a transaction to abort (examplesinclude but are not limited to MFENCE, LFENCE, SFENCE, RDTSC, RDTSCP,etc.).

The following instructions will abort transactional execution on anyimplementation:

-   -   XABORT    -   CPUID    -   PAUSE

In addition, in some implementations, the following instructions mayalways cause transactional aborts. These instructions are not expectedto be commonly used inside typical transactional regions. However,programmers must not rely on these instructions to force a transactionalabort, since whether they cause transactional aborts is implementationdependent.

-   -   Operations on X87 and MMX architecture state. This includes all        MMX and X87 instructions, including the FXRSTOR and FXSAVE        instructions.    -   Update to non-status portion of EFLAGS: CLI, STI, POPFD, POPFQ,        CLTS.    -   Instructions that update segment registers, debug registers        and/or control registers: MOV to DS/ES/FS/GS/SS, POP        DS/ES/FS/GS/SS, LDS, LES, LFS, LGS, LSS, SWAPGS, WRFSBASE,        WRGSBASE, LGDT, SGDT, LIDT, SIDT, LLDT, SLDT, LTR, STR, Far        CALL, Far JMP, Far RET, IRET, MOV to DRx, MOV to        CR0/CR2/CR3/CR4/CR8 and LMSW.    -   Ring transitions: SYSENTER, SYSCALL, SYSEXIT, and SYSRET.    -   TLB and Cacheability control: CLFLUSH, INVD, WBINVD, INVLPG,        INVPCID, and memory instructions with a non-temporal hint        (MOVNTDQA, MOVNTDQ, MOVNTI, MOVNTPD, MOVNTPS, and MOVNTQ).    -   Processor state save: XSAVE, XSAVEOPT, and XRSTOR.    -   Interrupts: INTn, INTO.    -   IO: IN, INS, REP INS, OUT, OUTS, REP OUTS and their variants.    -   VMX: VMPTRLD, VMPTRST, VMCLEAR, VMREAD, VMWRITE, VMCALL,        VMLAUNCH, VMRESUME, VMXOFF, VMXON, INVEPT, and INVVPID.    -   SMX: GETSEC.    -   UD2, RSM, RDMSR, WRMSR, HLT, MONITOR, MWAIT, XSETBV, VZEROUPPER,        MASKMOVQ, and V/MASKMOVDQU.        Runtime Considerations

In addition to the instruction-based considerations, runtime events maycause transactional execution to abort. These may be due to data accesspatterns or micro-architectural implementation features. The followinglist is not a comprehensive discussion of all abort causes.

Any fault or trap in a transaction that must be exposed to software willbe suppressed. Transactional execution will abort and execution willtransition to a non-transactional execution, as if the fault or trap hadnever occurred. If an exception is not masked, then that un-maskedexception will result in a transactional abort and the state will appearas if the exception had never occurred.

Synchronous exception events (#DE, #OF, #NP, #SS, #GP, #BR, #UD, #AC,#XF, #PF, #NM, #TS, #MF, #DB, #BP/INT3) that occur during transactionalexecution may cause an execution not to commit transactionally, andrequire a non-transactional execution. These events are suppressed as ifthey had never occurred. With HLE, since the non-transactional code pathis identical to the transactional code path, these events will typicallyre-appear when the instruction that caused the exception is re-executednon-transactionally, causing the associated synchronous events to bedelivered appropriately in the non-transactional execution. Asynchronousevents (NMI, SMI, INTR, IPI, PMI, etc.) occurring during transactionalexecution may cause the transactional execution to abort and transitionto a non-transactional execution. The asynchronous events will be pendedand handled after the transactional abort is processed.

Transactions only support write-back cacheable memory type operations. Atransaction may always abort if the transaction includes operations onany other memory type. This includes instruction fetches to UC memorytype.

Memory accesses within a transactional region may require the processorto set the Accessed and Dirty flags of the referenced page table entry.The behavior of how the processor handles this is implementationspecific. Some implementations may allow the updates to these flags tobecome externally visible even if the transactional region subsequentlyaborts. Some Intel TSX implementations may choose to abort thetransactional execution if these flags need to be updated. Further, aprocessor's page-table walk may generate accesses to its owntransactionally written but uncommitted state. Some Intel TSXimplementations may choose to abort the execution of a transactionalregion in such situations. Regardless, the architecture ensures that, ifthe transactional region aborts, then the transactionally written statewill not be made architecturally visible through the behavior ofstructures such as TLBs.

Executing self-modifying code transactionally may also causetransactional aborts. Programmers must continue to follow the Intelrecommended guidelines for writing self-modifying and cross-modifyingcode even when employing HLE and RTM. While an implementation of RTM andHLE will typically provide sufficient resources for executing commontransactional regions, implementation constraints and excessive sizesfor transactional regions may cause a transactional execution to abortand transition to a non-transactional execution. The architectureprovides no guarantee of the amount of resources available to dotransactional execution and does not guarantee that a transactionalexecution will ever succeed.

Conflicting requests to a cache line accessed within a transactionalregion may prevent the transaction from executing successfully. Forexample, if logical processor P0 reads line A in a transactional regionand another logical processor P1 writes line A (either inside or outsidea transactional region) then logical processor P0 may abort if logicalprocessor P1's write interferes with processor P0's ability to executetransactionally.

Similarly, if P0 writes line A in a transactional region and P1 reads orwrites line A (either inside or outside a transactional region), then P0may abort if P1's access to line A interferes with P0's ability toexecute transactionally. In addition, other coherence traffic may attimes appear as conflicting requests and may cause aborts. While thesefalse conflicts may happen, they are expected to be uncommon. Theconflict resolution policy to determine whether P0 or P1 aborts in theabove scenarios is implementation specific.

Generic Transaction Execution Embodiments:

According to “ARCHITECTURES FOR TRANSACTIONAL MEMORY”, a dissertationsubmitted to the Department of Computer Science and the Committee onGraduate Studies of Stanford University in partial fulfillment of therequirements for the Degree of Doctor of Philosophy, by Austen McDonald,June 2009, incorporated by reference herein in its entirety,fundamentally, there are three mechanisms needed to implement an atomicand isolated transactional region: versioning, conflict detection, andcontention management.

To make a transactional code region appear atomic, all the modificationsperformed by that transactional code region must be stored and keptisolated from other transactions until commit time. The system does thisby implementing a versioning policy. Two versioning paradigms exist:eager and lazy. An eager versioning system stores newly generatedtransactional values in place and stores previous memory values on theside, in what is called an undo-log. A lazy versioning system stores newvalues temporarily in what is called a write buffer, copying them tomemory only on commit. In either system, the cache is used to optimizestorage of new versions.

To ensure that transactions appear to be performed atomically, conflictsmust be detected and resolved. The two systems, i.e., the eager and lazyversioning systems, detect conflicts by implementing a conflictdetection policy, either optimistic or pessimistic. An optimistic systemexecutes transactions in parallel, checking for conflicts only when atransaction commits. A pessimistic system checks for conflicts at eachload and store. Similar to versioning, conflict detection also uses thecache, marking each line as either part of the read-set, part of thewrite-set, or both. The two systems resolve conflicts by implementing acontention management policy. Many contention management policies exist,some are more appropriate for optimistic conflict detection and some aremore appropriate for pessimistic. Described below are some examplepolicies.

Since each transactional memory (TM) system needs both versioningdetection and conflict detection, these options give rise to fourdistinct TM designs: Eager-Pessimistic (EP), Eager-Optimistic (EO),Lazy-Pessimistic (LP), and Lazy-Optimistic (LO). Table 2 brieflydescribes all four distinct TM designs.

FIGS. 1 and 2 depict an example of a multicore TM environment. FIG. 1shows many TM-enabled CPUs (CPU1 114 a, CPU2 114 b, etc.) on one die100, connected with an interconnect 122, under management of aninterconnect control 120 a, 120 b. Each CPU 114 a, 114 b (also known asa Processor) may have a split cache consisting of an Instruction Cache116 a, 116 b for caching instructions from memory to be executed and aData Cache 118 a, 118 b with TM support for caching data (operands) ofmemory locations to be operated on by CPU 114 a, 114 b (in FIG. 1, eachCPU 114 a, 114 b and its associated caches are referenced as 112 a, 112b). In an implementation, caches of multiple dies 100 are interconnectedto support cache coherency between the caches of the multiple dies 100.In an implementation, a single cache, rather than the split cache isemployed holding both instructions and data. In implementations, the CPUcaches are one level of caching in a hierarchical cache structure. Forexample each die 100 may employ a shared cache 124 to be shared amongstall the CPUs on the die 100. In another implementation, each die mayhave access to a shared cache 124, shared amongst all the processors ofall the dies 100.

FIG. 2 shows the details of an example transactional CPU environment112, having a CPU 114, including additions to support TM. Thetransactional CPU (processor) 114 may include hardware for supportingRegister Checkpoints 126 and special TM Registers 128. The transactionalCPU cache may have the MESI bits 130, Tags 140 and Data 142 of aconventional cache but also, for example, R bits 132 showing a line hasbeen read by the CPU 114 while executing a transaction and W bits 138showing a line has been written-to by the CPU 114 while executing atransaction.

A key detail for programmers in any TM system is how non-transactionalaccesses interact with transactions. By design, transactional accessesare screened from each other using the mechanisms above. However, theinteraction between a regular, non-transactional load with a transactioncontaining a new value for that address must still be considered. Inaddition, the interaction between a non-transactional store with atransaction that has read that address must also be explored. These areissues of the database concept isolation.

A TM system is said to implement strong isolation, sometimes calledstrong atomicity, when every non-transactional load and store acts likean atomic transaction. Therefore, non-transactional loads cannot seeuncommitted data and non-transactional stores cause atomicity violationsin any transactions that have read that address. A system where this isnot the case is said to implement weak isolation, sometimes called weakatomicity.

Strong isolation is often more desirable than weak isolation due to therelative ease of conceptualization and implementation of strongisolation. Additionally, if a programmer has forgotten to surround someshared memory references with transactions, causing bugs, then withstrong isolation, the programmer will often detect that oversight usinga simple debug interface because the programmer will see anon-transactional region causing atomicity violations. Also, programswritten in one model may work differently on another model.

Further, strong isolation is often easier to support in hardware TM thanweak isolation. With strong isolation, since the coherence protocolalready manages load and store communication between processors,transactions can detect non-transactional loads and stores and actappropriately. To implement strong isolation in software TransactionalMemory (TM), non-transactional code must be modified to include read-and write-barriers; potentially crippling performance. Although greateffort has been expended to remove many un-needed barriers, suchtechniques are often complex and performance is typically far lower thanthat of HTMs.

TABLE 2 Transactional Memory Design Space VERSIONING Lazy Eager CONFLICTOptimistic Storing updates in a write Not practical: waiting to updateDETECTION buffer; detecting conflicts at memory until commit time butcommit time. detecting conflicts at access time guarantees wasted workand provides no advantage Pessimistic Storing updates in a writeUpdating memory, keeping old buffer; detecting conflicts at values inundo log; detecting access time. conflicts at access time.

Table 2 illustrates the fundamental design space of transactional memory(versioning and conflict detection).

Eager-Pessimistic (EP)

This first TM design described below is known as Eager-Pessimistic. AnEP system stores its write-set “in place” (hence the name “eager”) and,to support rollback, stores the old values of overwritten lines in an“undo log”. Processors use the W 138 and R 132 cache bits to track readand write-sets and detect conflicts when receiving snooped loadrequests. Perhaps the most notable examples of EP systems in knownliterature are LogTM and UTM.

Beginning a transaction in an EP system is much like beginning atransaction in other systems: tm_begin( ) takes a register checkpoint,and initializes any status registers. An EP system also requiresinitializing the undo log, the details of which are dependent on the logformat, but often involve initializing a log base pointer to a region ofpre-allocated, thread-private memory, and clearing a log boundsregister.

Versioning: In EP, due to the way eager versioning is designed tofunction, the MESI 130 state transitions (cache line indicatorscorresponding to Modified, Exclusive, Shared, and Invalid code states)are left mostly unchanged. Outside of a transaction, the MESI 130 statetransitions are left completely unchanged. When reading a line inside atransaction, the standard coherence transitions apply (S (Shared)→S, I(Invalid)→S, or I→E (Exclusive)), issuing a load miss as needed, but theR 132 bit is also set. Likewise, writing a line applies the standardtransitions (S→M, E→I, I→M), issuing a miss as needed, but also sets theW 138 (Written) bit. The first time a line is written, the old versionof the entire line is loaded then written to the undo log to preserve itin case the current transaction aborts. The newly written data is thenstored “in-place,” over the old data.

Conflict Detection: Pessimistic conflict detection uses coherencemessages exchanged on misses, or upgrades, to look for conflicts betweentransactions. When a read miss occurs within a transaction, otherprocessors receive a load request; but they ignore the request if theydo not have the needed line. If the other processors have the neededline non-speculatively or have the line R 132 (Read), they downgradethat line to S, and in certain cases issue a cache-to-cache transfer ifthey have the line in MESI's 130 M or E state. However, if the cache hasthe line W 138, then a conflict is detected between the two transactionsand additional action(s) must be taken.

Similarly, when a transaction seeks to upgrade a line from shared tomodified (on a first write), the transaction issues an exclusive loadrequest, which is also used to detect conflicts. If a receiving cachehas the line non-speculatively, then the line is invalidated, and incertain cases a cache-to-cache transfer (M or E states) is issued. But,if the line is R 132 or W 138, a conflict is detected.

Validation: Because conflict detection is performed on every load, atransaction always has exclusive access to its own write-set. Therefore,validation does not require any additional work.

Commit: Since eager versioning stores the new version of data items inplace, the commit process simply clears the W 138 and R 132 bits anddiscards the undo log.

Abort: When a transaction rolls back, the original version of each cacheline in the undo log must be restored, a process called “unrolling” or“applying” the log. This is done during tm_discard( ) and must be atomicwith regard to other transactions. Specifically, the write-set muststill be used to detect conflicts: this transaction has the only correctversion of lines in its undo log, and requesting transactions must waitfor the correct version to be restored from that log. Such a log can beapplied using a hardware state machine or software abort handler.

Eager-Pessimistic has the characteristics of: Commit is simple and sinceit is in-place, very fast. Similarly, validation is a no-op. Pessimisticconflict detection detects conflicts early, thereby reducing the numberof “doomed” transactions. For example, if two transactions are involvedin a Write-After-Read dependency, then that dependency is detectedimmediately in pessimistic conflict detection. However, in optimisticconflict detection such conflicts are not detected until the writercommits.

Eager-Pessimistic also has the characteristics of: As described above,the first time a cache line is written, the old value must be written tothe log, incurring extra cache accesses. Aborts are expensive as theyrequire undoing the log. For each cache line in the log, a load must beissued, perhaps going as far as main memory before continuing to thenext line. Pessimistic conflict detection also prevents certainserializable schedules from existing.

Additionally, because conflicts are handled as they occur, there is apotential for livelock and careful contention management mechanisms mustbe employed to guarantee forward progress.

Lazy-Optimistic (LO)

Another popular TM design is Lazy-Optimistic (LO), which stores itswrite-set in a “write buffer” or “redo log” and detects conflicts atcommit time (still using the R 132 and W 138 bits).

Versioning: Just as in the EP system, the MESI protocol of the LO designis enforced outside of the transactions. Once inside a transaction,reading a line incurs the standard MESI transitions but also sets the R132 bit. Likewise, writing a line sets the W 138 bit of the line, buthandling the MESI transitions of the LO design is different from that ofthe EP design. First, with lazy versioning, the new versions of writtendata are stored in the cache hierarchy until commit while othertransactions have access to old versions available in memory or othercaches. To make available the old versions, dirty lines (M lines) mustbe evicted when first written by a transaction. Second, no upgrademisses are needed because of the optimistic conflict detection feature:if a transaction has a line in the S state, it can simply write to itand upgrade that line to an M state without communicating the changeswith other transactions because conflict detection is done at committime.

Conflict Detection and Validation: To validate a transaction and detectconflicts, LO communicates the addresses of speculatively modified linesto other transactions only when it is preparing to commit. Onvalidation, the processor sends one, potentially large, network packetcontaining all the addresses in the write-set. Data is not sent, butleft in the cache of the committer and marked dirty (M). To build thispacket without searching the cache for lines marked W, a simple bitvector is used, called a “store buffer,” with one bit per cache line totrack these speculatively modified lines. Other transactions use thisaddress packet to detect conflicts: if an address is found in the cacheand the R 132 and/or W 138 bits are set, then a conflict is initiated.If the line is found but neither R 132 nor W 138 is set, then the lineis simply invalidated, which is similar to processing an exclusive load.

To support transaction atomicity, these address packets must be handledatomically, i.e., no two address packets may exist at once with the sameaddresses. In an LO system, this can be achieved by simply acquiring aglobal commit token before sending the address packet. However, atwo-phase commit scheme could be employed by first sending out theaddress packet, collecting responses, enforcing an ordering protocol(perhaps oldest transaction first), and committing once all responsesare satisfactory.

Commit: Once validation has occurred, commit needs no special treatment:simply clear W 138 and R 132 bits and the store buffer. Thetransaction's writes are already marked dirty in the cache and othercaches' copies of these lines have been invalidated via the addresspacket. Other processors can then access the committed data through theregular coherence protocol.

Abort: Rollback is equally easy: because the write-set is containedwithin the local caches, these lines can be invalidated, then clear W138 and R 132 bits and the store buffer. The store buffer allows W linesto be found to invalidate without the need to search the cache.

Lazy-Optimistic has the characteristics of: Aborts are very fast,requiring no additional loads or stores and making only local changes.More serializable schedules can exist than found in EP, which allows anLO system to more aggressively speculate that transactions areindependent, which can yield higher performance. Finally, the latedetection of conflicts can increase the likelihood of forward progress.

Lazy-Optimistic also has the characteristics of: Validation takes globalcommunication time proportional to size of write set. Doomedtransactions can waste work since conflicts are detected only at committime.

Lazy-Pessimistic (LP)

Lazy-Pessimistic (LP) represents a third TM design option, sittingsomewhere between EP and LO: storing newly written lines in a writebuffer but detecting conflicts on a per access basis.

Versioning: Versioning is similar but not identical to that of LO:reading a line sets its R bit 132, writing a line sets its W bit 138,and a store buffer is used to track W lines in the cache. Also, dirty(M) lines must be evicted when first written by a transaction, just asin LO. However, since conflict detection is pessimistic, load exclusivesmust be performed when upgrading a transactional line from I, S→M, whichis unlike LO.

Conflict Detection: LP's conflict detection operates the same as EP's:using coherence messages to look for conflicts between transactions.

Validation: Like in EP, pessimistic conflict detection ensures that atany point, a running transaction has no conflicts with any other runningtransaction, so validation is a no-op.

Commit: Commit needs no special treatment: simply clear W 138 and R 132bits and the store buffer, like in LO.

Abort: Rollback is also like that of LO: simply invalidate the write-setusing the store buffer and clear the W and R bits and the store buffer.

Eager-Optimistic (EO)

The LP has the characteristics of: Like LO, aborts are very fast. LikeEP, the use of pessimistic conflict detection reduces the number of“doomed” transactions. Like EP, some serializable schedules are notallowed and conflict detection must be performed on each cache miss.

The final combination of versioning and conflict detection isEager-Optimistic (EO). EO may be a less than optimal choice for HTMsystems: since new transactional versions are written in-place, othertransactions have no choice but to notice conflicts as they occur (i.e.,as cache misses occur). But since EO waits until commit time to detectconflicts, those transactions become “zombies,” continuing to execute,wasting resources, yet are “doomed” to abort.

EO has proven to be useful in STMs and is implemented by Bartok-STM andMcRT. A lazy versioning STM needs to check its write buffer on each readto ensure that it is reading the most recent value. Since the writebuffer is not a hardware structure, this is expensive, hence thepreference for write-in-place eager versioning. Additionally, sincechecking for conflicts is also expensive in an STM, optimistic conflictdetection offers the advantage of performing this operation in bulk.

Contention Management

How a transaction rolls back once the system has decided to abort thattransaction has been described above, but, since a conflict involves twotransactions, the topics of which transaction should abort, how thatabort should be initiated, and when should the aborted transaction beretried need to be explored. These are topics that are addressed byContention Management (CM), a key component of transactional memory.Described below are policies regarding how the systems initiate abortsand the various established methods of managing which transactionsshould abort in a conflict.

Contention Management Policies

A Contention Management (CM) Policy is a mechanism that determines whichtransaction involved in a conflict should abort and when the abortedtransaction should be retried. For example, it is often the case thatretrying an aborted transaction immediately does not lead to the bestperformance. Conversely, employing a back-off mechanism, which delaysthe retrying of an aborted transaction, can yield better performance.STMs first grappled with finding the best contention management policiesand many of the policies outlined below were originally developed forSTMs.

CM Policies draw on a number of measures to make decisions, includingages of the transactions, size of read- and write-sets, the number ofprevious aborts, etc. The combinations of measures to make suchdecisions are endless, but certain combinations are described below,roughly in order of increasing complexity.

To establish some nomenclature, first note that in a conflict there aretwo sides: the attacker and the defender. The attacker is thetransaction requesting access to a shared memory location. Inpessimistic conflict detection, the attacker is the transaction issuingthe load or load exclusive. In optimistic, the attacker is thetransaction attempting to validate. The defender in both cases is thetransaction receiving the attacker's request.

An Aggressive CM Policy immediately and always retries either theattacker or the defender. In LO, Aggressive means that the attackeralways wins, and so Aggressive is sometimes called committer wins. Sucha policy was used for the earliest LO systems. In the case of EP,Aggressive can be either defender wins or attacker wins.

Restarting a conflicting transaction that will immediately experienceanother conflict is bound to waste work—namely interconnect bandwidthrefilling cache misses. A Polite CM Policy employs exponential backoff(but linear could also be used) before restarting conflicts. To preventstarvation, a situation where a process does not have resourcesallocated to it by the scheduler, the exponential backoff greatlyincreases the odds of transaction success after some n retries.

Another approach to conflict resolution is to randomly abort theattacker or defender (a policy called Randomized). Such a policy may becombined with a randomized backoff scheme to avoid unneeded contention.

However, making random choices, when selecting a transaction to abort,can result in aborting transactions that have completed “a lot of work”,which can waste resources. To avoid such waste, the amount of workcompleted on the transaction can be taken into account when determiningwhich transaction to abort. One measure of work could be a transaction'sage. Other methods include Oldest, Bulk TM, Size Matters, Karma, andPolka. Oldest is a simple timestamp method that aborts the youngertransaction in a conflict. Bulk TM uses this scheme. Size Matters islike Oldest but instead of transaction age, the number of read/writtenwords is used as the priority, reverting to Oldest after a fixed numberof aborts. Karma is similar, using the size of the write-set aspriority. Rollback then proceeds after backing off a fixed amount oftime. Aborted transactions keep their priorities after being aborted(hence the name Karma). Polka works like Karma but instead of backingoff a predefined amount of time, it backs off exponentially more eachtime.

Since aborting wastes work, it is logical to argue that stalling anattacker until the defender has finished their transaction would lead tobetter performance. Unfortunately, such a simple scheme easily leads todeadlock.

Deadlock avoidance techniques can be used to solve this problem. Greedyuses two rules to avoid deadlock. The first rule is, if a firsttransaction, T1, has lower priority than a second transaction, T0, or ifT1 is waiting for another transaction, then T1 aborts when conflictingwith T0. The second rule is, if T1 has higher priority than T0 and isnot waiting, then T0 waits until T1 commits, aborts, or starts waiting(in which case the first rule is applied). Greedy provides someguarantees about time bounds for executing a set of transactions. One EPdesign (LogTM) uses a CM policy similar to Greedy to achieve stallingwith conservative deadlock avoidance.

Example MESI coherency rules provide for four possible states in which acache line of a multiprocessor cache system may reside, M, E, S, and I,defined as follows:

Modified (M): The cache line is present only in the current cache, andis dirty; it has been modified from the value in main memory. The cacheis required to write the data back to main memory at some time in thefuture, before permitting any other read of the (no longer valid) mainmemory state. The write-back changes the line to the Exclusive state.

Exclusive (E): The cache line is present only in the current cache, butis clean; it matches main memory. It may be changed to the Shared stateat any time, in response to a read request. Alternatively, it may bechanged to the Modified state when writing to it.

Shared (S): Indicates that this cache line may be stored in other cachesof the machine and is “clean”; it matches the main memory. The line maybe discarded (changed to the Invalid state) at any time.

Invalid (I): Indicates that this cache line is invalid (unused).

TM coherency status indicators (R 132, W 138) may be provided for eachcache line, in addition to, or encoded in the MESI coherency bits. An R132 indicator indicates the current transaction has read from the dataof the cache line, and a W 138 indicator indicates the currenttransaction has written to the data of the cache line.

In another aspect of TM design, a system is designed using transactionalstore buffers. U.S. Pat. No. 6,349,361 titled “Methods and Apparatus forReordering and Renaming Memory References in a Multiprocessor ComputerSystem,” filed Mar. 31, 2000 and incorporated by reference herein in itsentirety, teaches a method for reordering and renaming memory referencesin a multiprocessor computer system having at least a first and a secondprocessor. The first processor has a first private cache and a firstbuffer, and the second processor has a second private cache and a secondbuffer. The method includes the steps of, for each of a plurality ofgated store requests received by the first processor to store a datum,exclusively acquiring a cache line that contains the datum by the firstprivate cache, and storing the datum in the first buffer. Upon the firstbuffer receiving a load request from the first processor to load aparticular datum, the particular datum is provided to the firstprocessor from among the data stored in the first buffer based on anin-order sequence of load and store operations. Upon the first cachereceiving a load request from the second cache for a given datum, anerror condition is indicated and a current state of at least one of theprocessors is reset to an earlier state when the load request for thegiven datum corresponds to the data stored in the first buffer.

The main implementation components of one such transactional memoryfacility are a transaction-backup register file for holdingpre-transaction GR (general register) content, a cache directory totrack the cache lines accessed during the transaction, a store cache tobuffer stores until the transaction ends, and firmware routines toperform various complex functions. In this section a detailedimplementation is described.

IBM zEnterprise EC12 Enterprise Server Embodiment

The IBM zEnterprise EC12 enterprise server introduces transactionalexecution (TX) in transactional memory, and is described in part in apaper, “Transactional Memory Architecture and Implementation for IBMSystem z” of Proceedings Pages 25-36 presented at MICRO-45, 1-5 Dec.2012, Vancouver, British Columbia, Canada, available from IEEE ComputerSociety Conference Publishing Services (CPS), which is incorporated byreference herein in its entirety.

Table 3 shows an example transaction. Transactions started with TBEGINare not assured to ever successfully complete with TEND, since they canexperience an aborting condition at every attempted execution, e.g., dueto repeating conflicts with other CPUs. This requires that the programsupport a fallback path to perform the same operationnon-transactionally, e.g., by using traditional locking schemes. Thisputs a significant burden on the programming and software verificationteams, especially where the fallback path is not automatically generatedby a reliable compiler.

TABLE 3 Example Transaction Code LHI R0,0 *initialize retry count=0 loopTBEGIN *begin transaction JNZ abort *go to abort code if CC1=0 LT R1,lock *load and test the fallback lock JNZ lckbzy *branch if lock busy .. . perform operation . . . TEND *end transaction . . . . . . . . . . .. lckbzy TABORT *abort if lock busy; this *resumes after TBEGIN abort JOfallback *no retry if CC=3 AHI R0, 1 *increment retry count CIJNL R0,6,fallback *give up after 6 attempts PPA R0, TX *random delay based onretry count . . . potentially wait for lock to become free . . . J loop*jump back to retry fallback OBTAIN lock *using Compare&Swap . . .perform operation . . . RELEASE lock . . . . . . . . . . . .

The requirement of providing a fallback path for aborted TransactionExecution (TX) transactions can be onerous. Many transactions operatingon shared data structures are expected to be short, touch only a fewdistinct memory locations, and use simple instructions only. For thosetransactions, the IBM zEnterprise EC12 introduces the concept ofconstrained transactions; under normal conditions, the CPU 114 (FIG. 2)assures that constrained transactions eventually end successfully,albeit without giving a strict limit on the number of necessary retries.A constrained transaction starts with a TBEGINC instruction and endswith a regular TEND. Implementing a task as a constrained ornon-constrained transaction typically results in very comparableperformance, but constrained transactions simplify software developmentby removing the need for a fallback path. IBM's Transactional Executionarchitecture is further described in z/Architecture, Principles ofOperation, Tenth Edition, SA22-7832-09 published September 2012 fromIBM, incorporated by reference herein in its entirety.

A constrained transaction starts with the TBEGINC instruction. Atransaction initiated with TBEGINC must follow a list of programmingconstraints; otherwise the program takes a non-filterableconstraint-violation interruption. Exemplary constraints may include,but not be limited to: the transaction can execute a maximum of 32instructions, all instruction text must be within 256 consecutive bytesof memory; the transaction contains only forward-pointing relativebranches (i.e., no loops or subroutine calls); the transaction canaccess a maximum of 4 aligned octowords (an octoword is 32 bytes) ofmemory; and restriction of the instruction-set to exclude complexinstructions like decimal or floating-point operations. The constraintsare chosen such that many common operations like doubly linkedlist-insert/delete operations can be performed, including the verypowerful concept of atomic compare-and-swap targeting up to 4 alignedoctowords. At the same time, the constraints were chosen conservativelysuch that future CPU implementations can assure transaction successwithout needing to adjust the constraints, since that would otherwiselead to software incompatibility.

TBEGINC mostly behaves like XBEGIN in TSX or TBEGIN on IBM's zEC12servers, except that the floating-point register (FPR) control and theprogram interruption filtering fields do not exist and the controls areconsidered to be zero. On a transaction abort, the instruction addressis set back directly to the TBEGINC instead of to the instruction after,reflecting the immediate retry and absence of an abort path forconstrained transactions.

Nested transactions are not allowed within constrained transactions, butif a TBEGINC occurs within a non-constrained transaction it is treatedas opening a new non-constrained nesting level just like TBEGIN would.This can occur, e.g., if a non-constrained transaction calls asubroutine that uses a constrained transaction internally.

Since interruption filtering is implicitly off, all exceptions during aconstrained transaction lead to an interruption into the operatingsystem (OS). Eventual successful finishing of the transaction relies onthe capability of the OS to page-in the at most 4 pages touched by anyconstrained transaction. The OS must also ensure time-slices long enoughto allow the transaction to complete.

TABLE 4 Transaction Code Example TBEGINC *begin constrained transaction. . . perform operation . . . TEND *end transaction

Table 4 shows the constrained-transactional implementation of the codein Table 3, assuming that the constrained transactions do not interactwith other locking-based code. No lock testing is shown therefore, butcould be added if constrained transactions and lock-based code weremixed.

When failure occurs repeatedly, software emulation is performed usingmillicode as part of system firmware. Advantageously, constrainedtransactions have desirable properties because of the burden removedfrom programmers.

With reference to FIG. 3, the IBM zEnterprise EC12 processor introducedthe transactional execution facility. The processor can decode 3instructions per clock cycle; simple instructions are dispatched assingle micro-ops, and more complex instructions are cracked intomultiple micro-ops. The micro-ops (Uops 232 b) are written into aunified issue queue 216, from where they can be issued out-of-order. Upto two fixed-point, one floating-point, two load/store, and two branchinstructions can execute every cycle. A Global Completion Table (GCT)232 holds every micro-op 232 b and a transaction nesting depth (TND) 232a. The GCT 232 is written in-order at decode time, tracks the executionstatus of each micro-op 232 b, and completes instructions when allmicro-ops 232 b of the oldest instruction group have successfullyexecuted.

The level 1 (L1) data cache 240 is a 96 KB (kilo-byte) 6-way associativecache with 256 byte cache-lines and 4 cycle use latency, coupled to aprivate 1 MB (mega-byte) 8-way associative 2nd-level (L2) data cache 268with 7 cycles use-latency penalty for L1 240 misses. The L1 240 cache isthe cache closest to a processor and Ln cache is a cache at the nthlevel of caching. Both L1 240 and L2 268 caches are store-through. Sixcores on each central processor (CP) chip share a 48 MB 3rd-levelstore-in cache, and six CP chips are connected to an off-chip 384 MB4th-level cache, packaged together on a glass ceramic multi-chip module(MCM). Up to 4 multi-chip modules (MCMs) can be connected to a coherentsymmetric multi-processor (SMP) system with up to 144 cores (not allcores are available to run customer workload).

Coherency is managed with a variant of the MESI protocol. Cache-linescan be owned read-only (shared) or exclusive; the L1 240 and L2 268 arestore-through and thus do not contain dirty lines. The L3 272 and L4caches (not shown) are store-in and track dirty states. Each cache isinclusive of all its connected lower level caches.

Coherency requests are called “cross interrogates” (XI) and are senthierarchically from higher level to lower-level caches, and between theL4s. When one core misses the L1 240 and L2 268 and requests the cacheline from its local L3 272, the L3 272 checks whether it owns the line,and if necessary sends an XI to the currently owning L2 268/L1 240 underthat L3 272 to ensure coherency, before it returns the cache line to therequestor. If the request also misses the L3 272, the L3 272 sends arequest to the L4 (not shown), which enforces coherency by sending XIsto all necessary L3s under that L4, and to the neighboring L4s. Then theL4 responds to the requesting L3 which forwards the response to the L2268/L1 240.

Note that due to the inclusivity rule of the cache hierarchy, sometimescache lines are XI'ed from lower-level caches due to evictions onhigher-level caches caused by associativity overflows from requests toother cache lines. These XIs can be called “LRU XIs”, where LRU standsfor least recently used.

Making reference to yet another type of XI requests, Demote-XIstransition cache-ownership from exclusive into read-only state, andExclusive-XIs transition cache ownership from exclusive into invalidstate. Demote-XIs and Exclusive-XIs need a response back to the XIsender. The target cache can “accept” the XI, or send a “reject”response if it first needs to evict dirty data before accepting the XI.The L1 240/L2 268 caches are store through, but may reject demote-XIsand exclusive XIs if they have stores in their store queues that need tobe sent to L3 before downgrading the exclusive state. A rejected XI willbe repeated by the sender. Read-only-XIs are sent to caches that own theline read-only; no response is needed for such XIs since they cannot berejected. The details of the SMP protocol are similar to those describedfor the IBM z10 by P. Mak, C. Walters, and G. Strait, in “IBM System z10processor cache subsystem microarchitecture”, IBM Journal of Researchand Development, Vol 53:1, 2009, which is incorporated by referenceherein in its entirety.

Transactional Instruction Execution

FIG. 3 depicts example components of an example transactional executionenvironment, including a CPU and caches/components with which itinteracts (such as those depicted in FIGS. 1 and 2). The instructiondecode unit 208 (IDU) keeps track of the current transaction nestingdepth 212 (TND). When the IDU 208 receives a TBEGIN instruction, thenesting depth 212 is incremented, and conversely decremented on TENDinstructions. The nesting depth 212 is written into the GCT 232 forevery dispatched instruction. When a TBEGIN or TEND is decoded on aspeculative path that later gets flushed, the IDU's 208 nesting depth212 is refreshed from the youngest GCT 232 entry that is not flushed.The transactional state is also written into the issue queue 216 forconsumption by the execution units, mostly by the Load/Store Unit (LSU)280, which also has an effective address calculator 236 included in theLSU 280. The TBEGIN instruction may specify a transaction diagnosticblock (TDB) for recording status information, should the transactionabort before reaching a TEND instruction.

Similar to the nesting depth, the IDU 208/GCT 232 collaboratively trackthe access register/floating-point register (AR/FPR) modification masksthrough the transaction nest; the IDU 208 can place an abort requestinto the GCT 232 when an AR/FPR-modifying instruction is decoded and themodification mask blocks that. When the instruction becomesnext-to-complete, completion is blocked and the transaction aborts.Other restricted instructions are handled similarly, including TBEGIN ifdecoded while in a constrained transaction, or exceeding the maximumnesting depth.

An outermost TBEGIN is cracked into multiple micro-ops depending on theGR-Save-Mask; each micro-op 232 b (including, for example uop 0, uop 1,and uop2) will be executed by one of the two fixed point units (FXUs)220 to save a pair of GRs 228 into a special transaction-backup registerfile 224, that is used to later restore the GR 228 content in case of atransaction abort. Also the TBEGIN spawns micro-ops 232 b to perform anaccessibility test for the TDB if one is specified; the address is savedin a special purpose register for later usage in the abort case. At thedecoding of an outermost TBEGIN, the instruction address and theinstruction text of the TBEGIN are also saved in special purposeregisters for a potential abort processing later on.

TEND and NTSTG are single micro-op 232 b instructions; NTSTG(non-transactional store) is handled like a normal store except that itis marked as non-transactional in the issue queue 216 so that the LSU280 can treat it appropriately. TEND is a no-op at execution time, theending of the transaction is performed when TEND completes.

As mentioned, instructions that are within a transaction are marked assuch in the issue queue 216, but otherwise execute mostly unchanged; theLSU 280 performs isolation tracking as described in the next section.

Since decoding is in-order, and since the IDU 208 keeps track of thecurrent transactional state and writes it into the issue queue 216 alongwith every instruction from the transaction, execution of TBEGIN, TEND,and instructions before, within, and after the transaction can beperformed out-of order. It is even possible (though unlikely) that TENDis executed first, then the entire transaction, and lastly the TBEGINexecutes. Program order is restored through the GCT 232 at completiontime. The length of transactions is not limited by the size of the GCT232, since general purpose registers (GRs) 228 can be restored from thebackup register file 224.

During execution, the program event recording (PER) events are filteredbased on the Event Suppression Control, and a PER TEND event is detectedif enabled. Similarly, while in transactional mode, a pseudo-randomgenerator may be causing the random aborts as enabled by the TransactionDiagnostics Control.

Tracking for Transactional Isolation

The Load/Store Unit 280 tracks cache lines that were accessed duringtransactional execution, and triggers an abort if an XI from another CPU(or an LRU-XI) conflicts with the footprint. If the conflicting XI is anexclusive or demote XI, the LSU 280 rejects the XI back to the L3 272 inthe hope of finishing the transaction before the L3 272 repeats the XI.This “stiff-arming” is very efficient in highly contended transactions.In order to prevent hangs when two CPUs stiff-arm each other, aXI-reject counter is implemented, which triggers a transaction abortwhen a threshold is met.

The L1 cache directory 240 is traditionally implemented with staticrandom access memories (SRAMs). For the transactional memoryimplementation, the valid bits 244 (64 rows×6 ways) of the directoryhave been moved into normal logic latches, and are supplemented with twomore bits per cache line: the TX-read 248 and TX-dirty 252 bits.

The TX-read 248 bits are reset when a new outermost TBEGIN is decoded(which is interlocked against a prior still pending transaction). TheTX-read 248 bit is set at execution time by every load instruction thatis marked “transactional” in the issue queue. Note that this can lead toover-marking if speculative loads are executed, for example on amispredicted branch path. The alternative of setting the TX-read 248 bitat load completion time was too expensive for silicon area, sincemultiple loads can complete at the same time, requiring many read-portson the load-queue.

Stores execute the same way as in non-transactional mode, but atransaction mark is placed in the store queue (STQ) 260 entry of thestore instruction. At write-back time, when the data from the STQ 260 iswritten into the L1 240, the TX-dirty bit 252 in the L1-directory 256 isset for the written cache line. Store write-back into the L1 240 occursonly after the store instruction has completed, and at most one store iswritten back per cycle. Before completion and write-back, loads canaccess the data from the STQ 260 by means of store-forwarding; afterwrite-back, the CPU 114 (FIG. 2) can access the speculatively updateddata in the L1 240. If the transaction ends successfully, the TX-dirtybits 252 of all cache-lines are cleared, and also the TX-marks of notyet written stores are cleared in the STQ 260, effectively turning thepending stores into normal stores.

On a transaction abort, all pending transactional stores are invalidatedfrom the STQ 260, even those already completed. All cache lines thatwere modified by the transaction in the L1 240, that is, have theTX-dirty bit 252 on, have their valid bits turned off, effectivelyremoving them from the L1 240 cache instantaneously.

The architecture requires that before completing a new instruction, theisolation of the transaction read- and write-set is maintained. Thisisolation is ensured by stalling instruction completion at appropriatetimes when XIs are pending; speculative out-of order execution isallowed, optimistically assuming that the pending XIs are to differentaddresses and not actually cause a transaction conflict. This designfits very naturally with the XI-vs-completion interlocks that areimplemented on prior systems to ensure the strong memory ordering thatthe architecture requires.

When the L1 240 receives an XI, L1 240 accesses the directory to checkvalidity of the XI'ed address in the L1 240, and if the TX-read bit 248is active on the XI'ed line and the XI is not rejected, the LSU 280triggers an abort. When a cache line with active TX-read bit 248 isLRU'ed from the L1 240, a special LRU-extension vector remembers foreach of the 64 rows of the L1 240 that a TX-read line existed on thatrow. Since no precise address tracking exists for the LRU extensions,any non-rejected XI that hits a valid extension row the LSU 280 triggersan abort. Providing the LRU-extension effectively increases the readfootprint capability from the L1-size to the L2-size and associativity,provided no conflicts with other CPUs 114 (FIGS. 1 and 2) against thenon-precise LRU-extension tracking causes aborts.

The store footprint is limited by the store cache size (the store cacheis discussed in more detail below) and thus implicitly by the L2 268size and associativity. No LRU-extension action needs to be performedwhen a TX-dirty 252 cache line is LRU'ed from the L1 240.

Store Cache

In prior systems, since the L1 240 and L2 268 are store-through caches,every store instruction causes an L3 272 store access; with now 6 coresper L3 272 and further improved performance of each core, the store ratefor the L3 272 (and to a lesser extent for the L2 268) becomesproblematic for certain workloads. In order to avoid store queuingdelays, a gathering store cache 264 had to be added, that combinesstores to neighboring addresses before sending them to the L3 272.

For transactional memory performance, it is acceptable to invalidateevery TX-dirty 252 cache line from the L1 240 on transaction aborts,because the L2 268 cache is very close (7 cycles L1 240 miss penalty) tobring back the clean lines. However, it would be unacceptable forperformance (and silicon area for tracking) to have transactional storeswrite the L2 268 before the transaction ends and then invalidate alldirty L2 268 cache lines on abort (or even worse on the shared L3 272).

The two problems of store bandwidth and transactional memory storehandling can both be addressed with the gathering store cache 264. Thecache 264 is a circular queue of 64 entries, each entry holding 128bytes of data with byte-precise valid bits. In non-transactionaloperation, when a store is received from the LSU 280, the store cache264 checks whether an entry exists for the same address, and if sogathers the new store into the existing entry. If no entry exists, a newentry is written into the queue, and if the number of free entries fallsunder a threshold, the oldest entries are written back to the L2 268 andL3 272 caches.

When a new outermost transaction begins, all existing entries in thestore cache are marked closed so that no new stores can be gathered intothem, and eviction of those entries to L2 268 and L3 272 is started.From that point on, the transactional stores coming out of the LSU 280STQ 260 allocate new entries, or gather into existing transactionalentries. The write-back of those stores into L2 268 and L3 272 isblocked, until the transaction ends successfully; at that pointsubsequent (post-transaction) stores can continue to gather intoexisting entries, until the next transaction closes those entries again.

The store cache 264 is queried on every exclusive or demote XI, andcauses an XI reject if the XI compares to any active entry. If the coreis not completing further instructions while continuously rejecting XIs,the transaction is aborted at a certain threshold to avoid hangs.

The LSU 280 requests a transaction abort when the store cache 264overflows. The LSU 280 detects this condition when it tries to send anew store that cannot merge into an existing entry, and the entire storecache 264 is filled with stores from the current transaction. The storecache 264 is managed as a subset of the L2 268: while transactionallydirty lines can be evicted from the L1 240, they have to stay residentin the L2 268 throughout the transaction. The maximum store footprint isthus limited to the store cache size of 64×128 bytes, and it is alsolimited by the associativity of the L2 268. Since the L2 268 is 8-wayassociative and has 512 rows, it is typically large enough to not causetransaction aborts.

If a transaction aborts, the store cache 264 is notified and all entriesholding transactional data are invalidated. The store cache 264 also hasa mark per doubleword (8 bytes) whether the entry was written by a NTSTGinstruction—those doublewords stay valid across transaction aborts.

Millicode-Implemented Functions

Traditionally, IBM mainframe server processors contain a layer offirmware called millicode which performs complex functions like certainCISC instruction executions, interruption handling, systemsynchronization, and RAS. Millicode includes machine dependentinstructions as well as instructions of the instruction set architecture(ISA) that are fetched and executed from memory similarly toinstructions of application programs and the operating system (OS).Firmware resides in a restricted area of main memory that customerprograms cannot access. When hardware detects a situation that needs toinvoke millicode, the instruction fetching unit 204 switches into“millicode mode” and starts fetching at the appropriate location in themillicode memory area. Millicode may be fetched and executed in the sameway as instructions of the instruction set architecture (ISA), and mayinclude ISA instructions.

For transactional memory, millicode is involved in various complexsituations. Every transaction abort invokes a dedicated millicodesub-routine to perform the necessary abort operations. Thetransaction-abort millicode starts by reading special-purpose registers(SPRs) holding the hardware internal abort reason, potential exceptionreasons, and the aborted instruction address, which millicode then usesto store a TDB if one is specified. The TBEGIN instruction text isloaded from an SPR to obtain the GR-save-mask, which is needed formillicode to know which GRs 238 to restore.

The CPU 114 (FIG. 2) supports a special millicode-only instruction toread out the backup-GRs 224 and copy them into the main GRs 228. TheTBEGIN instruction address is also loaded from an SPR to set the newinstruction address in the PSW to continue execution after the TBEGINonce the millicode abort sub-routine finishes. That PSW may later besaved as program-old PSW in case the abort is caused by a non-filteredprogram interruption.

The TABORT instruction may be millicode implemented; when the IDU 208decodes TABORT, it instructs the instruction fetch unit to branch intoTABORT's millicode, from which millicode branches into the common abortsub-routine.

The Extract Transaction Nesting Depth (ETND) instruction may also bemillicoded, since it is not performance critical; millicode loads thecurrent nesting depth out of a special hardware register and places itinto a GR 228. The PPA instruction is millicoded; it performs theoptimal delay based on the current abort count provided by software asan operand to PPA, and also based on other hardware internal state.

For constrained transactions, millicode may keep track of the number ofaborts. The counter is reset to 0 on successful TEND completion, or ifan interruption into the OS occurs (since it is not known if or when theOS will return to the program). Depending on the current abort count,millicode can invoke certain mechanisms to improve the chance of successfor the subsequent transaction retry. The mechanisms involve, forexample, successively increasing random delays between retries, andreducing the amount of speculative execution to avoid encounteringaborts caused by speculative accesses to data that the transaction isnot actually using. As a last resort, millicode can broadcast to otherCPUs 114 (FIG. 2) to stop all conflicting work, retry the localtransaction, before releasing the other CPUs 114 to continue normalprocessing. Multiple CPUs 114 must be coordinated to not causedeadlocks, so some serialization between millicode instances ondifferent CPUs 114 is required.

In various embodiments, caches, for example, data caches 118, sharedcache 124, L1 cache 240, L2 cache 268, and L3 cache 272, may comprisecache entries, each of which may be capable of holding a cache line ofdata. In these embodiments, a cache line of memory data may be fetchedinto a cache entry, and the cache entry may be marked as a read-set orwrite-set cache line of a TM transaction. As referenced herein, theterms “cache entry” and “cache line” may be used interchangeably.

As described above, during TM transactional execution, memory dataaccessing instructions executed by a CPU core that fetch memoryoperands, such as a load-and-add instruction, may mark the cache linesinto which the memory operands are fetched as read-set cache lines ofthe transaction, via, for example, tx-read bit 248. This allows, forexample, the CPU core to determine if a conflicting memory data accesson another CPU core occurs during the transactional execution. Aconflicting XI received, for example, from another CPU core requestingexclusive ownership of data in a read-set cache line of a transactionmay cause the transaction to be aborted.

For ease of description, cache coherency requests, such as XIs, aredescribed as being received from other CPU cores or processors. However,those of skill in the art will recognize that XIs may be received fromother threads of a process, from other processes executing on a singleprocessor, from other processors of a multi-processor CPU core, or fromother CPU cores accessing common memory. Furthermore, other cachecoherency protocols are well known in the art that may not use XIs.Embodiments using such protocols are consistent with the teaching hereinwherein conflicts are detected using such protocols.

TM systems may require hardware to maintain state information aboutevery memory location accessed by a TM transaction until the transactioncompletes. The hardware resources needed to maintain this informationmay increase with transaction size, which may result in a limit to thetransaction size due to the number of memory locations a largetransaction may access. For example, a dynamically sized data structure,such as a linked list, may potentially need to access a large number ofmemory locations during a single atomic transaction to, for example,scan down the linked list. In certain TM system designs, a transactionmay be limited in size because there may be a maximum allowed number ofread tags 132 the transaction can allocate.

In addition, if a large number of cache lines 142 are marked as read-setcache lines, an unacceptable level of false transaction failures mayoccur when XIs to read-set cache lines are to data addresses in thecache lines that were not in fact read by the transaction. Also, markedcache lines may not be moved out of cache until the transactionalexecution completes, which may also cause performance problems.

In addition to the hardware resources needed to maintain stateinformation for all memory location accessed by a large transaction,increasing resources may be required to complete the comparisons of atransaction's read and write sets against the write sets of concurrentlyexecuting transactions as the number of read tags for the transactionincreases, and the number of allowed concurrently executing transactionsincreases.

In a TM system with a defined maximum number of cache lines for atransaction that can be marked as read-set cache lines, it may bebeneficial, when the maximum is reached, to have the system identifycandidate read-tagged cache lines for eviction based on whether the lineis a non-conflict cache line, meaning, for example, that the program hasdetermined that there will be no stores by other processors to theaddress locations in the cache line.

Read and Track/Read Don't Track

In embodiments of the disclosure, two new instruction related schemeswithin a TM transactional execution are defined. The first instructionscheme is referred to as Read and Track (RT). The RT instruction schemeallows non-conflict memory addresses to be identified for TM monitoringwithout reference to cache line read-set bits. Non-conflict addressescan be, for example, addresses that may contain constants or other fixeddata, and other addresses that may be accessed by a TM transaction thatshould not be stored to. For example, an RT address range may define therange for page 1 of the program address space as a non-conflict range,and this range can be reserved for storing constants, fixed data, andother read-only non-conflict data. The RT addresses may be identified,for example, in a parameter of the TBEGIN instruction or of a newRT_DEFINE instruction, containing the address of an RT register orregister block of user-specified address ranges. In one embodiment, theRT registers may be contained, for example, within the cache controlsubsystem of data cache 118. For example, the RT registers can be alinked list in which each register can contain a start and end addressof an address range of non-conflict addresses. In an embodiment, RTaddress ranges may be defined at the start of a TM transaction andremain in effect until the transaction completes.

In certain embodiments, when data at an address in an RT address rangeis fetched into a cache line, for example, a line in cache 118, thecache line containing the fetched data is not marked as read-set cacheline of the transaction. However, during execution of a TM transaction,addresses in an RT address range are effectively treated as read-setaddresses for cache coherency purposes. XIs from other CPU cores may becompared to the RT address ranges stored in the RT registers, as well asthe addresses associated with the read-set of the TM transaction, todetermine if a cache coherency conflict exists. For example, duringexecution of a TM transaction, if an XI from another CPU core requestsexclusive ownership of data at a memory address in an RT address range,the TM transaction may be aborted.

In another embodiment, a cache line containing data fetched from amemory address in an RT address range may be marked as a read-set cacheline of the transaction. In this embodiment, XIs from other CPU coresmay be compared to the addresses associated with the read-set of the TMtransaction, and not to the RT address ranges stored in the RTregisters. However, cache evictions may be based at least in part onwhether a read-set cache line contains data from a memory address in anRT address range. For example, if the maximum number of cache lines forthe transaction have been marked as read-set cache lines, a read-setcache line containing data from a memory address in an RT address rangemay take precedence for eviction over a cache line that does not containdata from a memory address in an RT address range.

The second instruction scheme within a TM transaction is referred to asRead-Don't-Track (RDT). The RDT instruction scheme allows addresses towhich data should not be written during execution of a TM transaction(non-conflict addresses)—for example, addresses that containconstants—to be flagged such that when data from these addresses isfetched, the cache lines into which the data is fetched may not berequired to be marked as a read-set cache lines.

RDT addresses may include, for example, addresses that containconstants, fixed data, and other read-only data, or that may beprotected against update through other established protectionfacilities. For example, programming conventions dictate that constants,fixed data, and other read-only data should not be stored to. Assumingthese conventions will be followed, marking the cache line containingthis data as read-set cache lines of a TM transaction may not benecessary. Similarly, if data is protected through establishedprotection facilities, for example, through key-controlled protection,access-list-controlled protection, or Dynamic Address Translation (DAT)protection, it may not be necessary to mark the cache lines containingthis data as read set cache lines of a transaction. In an embodiment,cache lines containing such data may, however, be included in atransaction's read-set cache lines as an extra measure of protectionagainst errant writes.

RDT addresses may be identified, for example, in a parameter of theTBEGIN instruction, or of a new RDT_DEFINE instruction, containing theaddress of a register or register block of user-specified non-conflictaddress ranges. RDT address ranges may be defined at the start of a TMtransaction and remain in effect until the transaction completes.

In an embodiment, when data at an RDT address is fetched into a cacheline, for example, a cache line of a cache 118, the cache linecontaining the fetched RDT data is not marked as a read-set cache lineof the transaction. Because data from RDT addresses should be read-only,there should not be conflicting XIs to such data stored in cache. Cacheevictions to allow for a new fetch may be based at least in part on adata in cache line being from an RDT address range.

FIG. 4 shows an embodiment of a multicore TM environment 400, inaccordance with an embodiment of the present disclosure, that includes aread_track (RT)/read_don't_track (RDT) registers 402A. TM multicoreenvironment 400 is based on the environment of FIGS. 1, 2, and 3,described above, and includes CPU cores 112A and 112B, shared cache 124,and main memory 404, all interconnected over interconnect 122. CPU core112A includes CPU1 114A, instruction cache 116A, data cache 118A, RT/RDTregisters 402A, and interconnect control 120A. CPU core 112B may beconfigured substantially the same as CPU core 112A. Components of TMmulticore environment 400 having like reference numerals as componentsdescribed above with reference to the environment of FIGS. 1 and 2operate in substantially the same manner. Although a single CPU core112A is illustrated, the embodiments and concepts described may apply toimplementations that include multiple CPU cores coupled to interconnect122.

In an embodiment, during execution of a TM transaction, IDU 208 maydecode an instruction that identifies an RT or RDT address range, forexample, an RT_DEFINE instruction, an RDT_DEFINE instruction, and/or aparameter of a TBEGIN instruction that defines an RT or RDT addressrange. IDU 208 may then write the RT or RDT address range into, forexample, one or more registers of RT/RDT registers 402A. When a memorydata accessing instruction is executed that causes a fetch of memorydata, for example, data at an address in main memory 404, into, forexample, data cache 118A, the data cache compares the address of thememory operand to the address ranges identified in RT/RDT registers402A.

During execution of a TM transaction, if the memory operand address isin an address range identified in an RT address register 402A, then, inone embodiment, data at the memory operand address is fetched into, forexample, a cache line of data cache 118A, and the line is not marked asa read-set cache line of the TM transaction. If, for example, an XIrequesting exclusive ownership of data is received by CPU core 112Afrom, for example, CPU core 112B, via interconnect 122, then data cache118A may compare the XI address to the RT address ranges identified inRT registers 402A. If the XI address is in an address range identifiedin a RT register 402A, then a conflict exists, and the TM transactionexecuting on CPU core 112A may abort.

In another embodiment, during execution of a TM transaction, memoryoperands that are fetched into, for example, data cache 118A, are markedas read-set cache lines of the transaction regardless of whether theaddress of the memory operand is in an address range identified in an RTaddress register. In this embodiment, XI conflicts can also bedetermined based on whether the XI address conflicts with an address inan address range identified in an RT address register. However, if, forexample, a cache line in data cache 118A must be evicted in order toallow a fetch of a memory operand from, for example, main memory 404,into the data cache, a cache line marked as a read-set cache line of thetransaction whose addresses are in an address range identified in an RTregister 402A, may be identified as a candidate cache line for eviction.

Cache Eviction Based on Read Tags at Eviction Time

FIG. 5 is a flowchart depicting operational steps for performing anaspect of cache line evictions during transactional execution of aprocess, in accordance with an embodiment of the disclosure. In general,the operational steps define a process that occurs when a predefinedmaximum number of cache lines for a transaction have been marked asread-set cache lines, for example, by setting R-bit 132, and that takesinto account whether a read-set cache line is a non-conflict cache lineby being in a non-conflict RT or RDT address range. If so, the cacheline can be a candidate for invalidation so as to allow another loadaddress to be fetched and flagged as a read-set member.

Upon execution, for example, of a TM load instruction, cache is checkedto determine if the data at the load address is already in cache, forexample L1 cache 240 (decision step 502). If the data at the loadaddress is in cache (decision step 502, “Y” branch), the MESI bits 130,for example, may be updated as appropriate (step 420), and the loadends. For ease of description, it is assumed here that if the loadaddress data is already in cache, the cache line containing the loadaddress data is a read-set cache line of the transaction.

If the load address data is not in cache (decision step 502, “N”branch), the system then determines if a predefined threshold number ofcache lines for the transaction have been marked as read-set cache lines(decision step 504). If the threshold number of read-set cache lines forthe transaction has not been reached (decision step 504, “N” branch),the data at the load address is fetched into a cache line 142 (step516), the read-bit 132/248 on the cache line may be set to indicate thatthe line is a read-set cache line of the transaction (step 518), theMESI bits 130 may be set as appropriate (step 520), and the load ends.For ease of description, it is assumed here that a cache line isavailable, or can be made available, into which the data at the loadaddress can be fetched.

If the predefined maximum number of cache lines for a transaction havebeen marked as read-set cache lines (decision step 504, “Y” branch), thesystem determines if any cache lines in the congruence class of the loadaddress can be identified as read-set cache lines of a transaction andcontain data from a defined RDT range (decision step 506). As describedabove, an address in an RDT address range should not be written to(non-conflict address), or may be protected against update throughestablished protection facilities, for example, through key-controlledprotection, access-list-controlled protection, and Dynamic AddressTranslation (DAT) protection. As such, there may be no need to monitorif an address in an RDT address range appears in other concurrent TMtransactions' write-sets.

If a cache line in the congruence class of the load address can beidentified as a read-set cache line of a transaction and has beenindicated as RDT (decision step 506, “Y” branch), the cache line may bereplaced by invalidating the cache line (step 514), which at leastserves the purpose of freeing up a read-set cache line, the data at theload address may be fetched into the cache line 142 (step 516), theread-bit 132/248 on the cache line may be set to indicate that the lineis a read-set cache line of the transaction (step 518), the MESI bits130 may be set as appropriate (step 520), and the load ends.

If no cache line in the congruence class of the load address can beidentified as a read-set cache line of a transaction and has beenindicated as RDT (decision step 506, “N” branch), the system determinesif any cache lines in the congruence class of the load address can beidentified as a read-set cache line of a transaction and contain datafrom a defined RT address range (decision step 508). As described above,an address in an RT address range can be monitored via one or more RTregisters.

If a cache line in the congruence class of the load address can beidentified as a read-set cache line of a transaction and has beenindicated as RT (decision step 508, “Y” branch), the cache line may bereplaced by invalidating the cache line (step 514), the data at the loadaddress may be fetched into a cache cache line 142 (step 516), theread-bit 132/248 on the cache line may be set to indicate that the lineis a read-set cache line of the transaction (step 518), the MESI bits130 may be set as appropriate (step 520), and the load ends.

If no cache line in the congruence class of the load address can beidentified as a read-set cache line of a transaction and has beenindicated as RT (decision step 508, “N” branch), the system determinesif any cache lines in the congruence class of the load address can beidentified as protected against update through established protectionfacilities, for example, through key-controlled protection,access-list-controlled protection, and Dynamic Address Translation (DAT)protection (decision step 510).

If a cache line in the congruence class of the load address can beidentified as containing data that is protected against update throughestablished protection facilities (decision step 510, “Y” branch), thecache line may be replaced by invalidating the cache line (step 514),the data at the load address may be fetched into the cache line 142(step 516), the read-bit 132/248 on the cache line may be set toindicate that the line is a read-set cache line of the transaction (step518), the MESI bits 130 may be set as appropriate (step 520), and theload ends.

If the maximum number of cache lines for the transaction have beenmarked as read-set cache lines, and no cache lines can be identified asa read-set cache line of a transaction and either in an RDT addressrange, an RT address range, or identified as protected against updatethrough established protection facilities, the transaction may beaborted (step 512). In certain embodiments, although no cache lines maybe available to receive data from the load address, the data may bewritten to one or more general registers and made available to thetransaction.

In certain embodiments, although the load address data is in cache (seestep 502, “Y” branch), the cache line containing the load address datamay not be a read-set cache line of the transaction. In this case, ifthe maximum number of cache lines for the transaction have been markedas read-set cache lines (see decision step 504, “Y” branch), an analysissimilar to determinations described in relation to steps 506 through 510may be performed.

In certain embodiments, although the load address data is not in cache(see step 502, “N” branch) and the maximum number of cache lines markedas read-set cache lines for the transaction has not been reached (seedecision step 504, “N” branch), it may be determined if a cache line isavailable, or can be made available, into which the data at the loadaddress can be fetched. If no cache lines are available, or can be madeavailable, into which the data at the load address can be fetched, thetransaction, for example, may be aborted.

In certain embodiments, although the load address data is in cache, thecache line containing the load address data may not be marked as aread-set cache line of the transaction. In this situation, if themaximum number of read-set cache lines for the transaction has beenreached, an analysis similar to determinations described in relation tosteps 506 through 510 may be performed to determine if a cache line inthe congruence class can be invalidated so as to allow for the load tocomplete. If no cache lines in the congruence class can be madeavailable into which the data at the load address can be fetched, thetransaction, for example, may be aborted.

In certain embodiments, although the maximum number of cache lines for atransaction have not been marked as read-set cache lines, all cachelines of a congruence class may be marked as read-set cache lines forthe transaction, and a load is directed to data that would be written toa cache line in the congruence class. In this situation, an analysissimilar to determinations described in relation to steps 506 through 510may be performed to determine if a cache line in the congruence classcan be invalidated so as to allow for the load to complete. If no cachelines in the congruence class can be made available into which the dataat the load address can be fetched, the transaction, for example, may beaborted.

Cache Eviction Based on Read Tags at Install Time

FIG. 6 is a flowchart depicting operational steps for performing asecond aspect of cache line evictions during transactional execution ofa process, in accordance with an embodiment of the disclosure. Ingeneral, the operational steps define a process that occurs when data ata load address is loaded into cache, and takes into account whether aread-set cache line is monitored by being in a non-conflict RT addressrange, not monitored by being in a non-conflict RDT address range, or isprotected against update through an established protection facility.

Upon execution, for example, of a TM load instruction, it is determinedwhether data at the load address in the instruction is to be monitored,i.e., whether the cache line to receive the data fetched from the loadaddress will be marked as a read-set cache line of the transaction(decision step 602). This is the typical case for loads occurring in aTM transaction. However, for example, in certain embodiments, the opcode of the load instruction can indicate whether the load address willbe marked as a read-set cache line (see, e.g., Tremblay 2003).

If the cache line receiving the data at the memory address in the loadinstruction will not be marked as a read-set cache line (decision step602, “N” branch), the system determines if there are any open cachelines in the congruence class of the load address (decision step 614).For example, an open cache line can be a line that may be invalid. Ifthere are open cache lines in the congruence class of the load address(decision step 614, “Y” branch), the data at the load address may befetched into the open cache line, the appropriate MESI bits 130 may beset, and the cache line is not marked as a read-set cache line of thetransaction, i.e., read-bit 132/248 is not set (step 622).

If there are not open cache lines in the congruence class of the loadaddress (decision step 614, “N” branch), the system determines if anycache lines in the congruence class of the load address do not havetheir read-bit or write-bit, 132/244 or 138/252, respectively, set(decision step 616). If a cache line in the congruence class of the loadaddress does not have a read-bit or write-bit, 132/244 or 138/252,respectively, set (decision step 616, “Y” branch), the cache line may beinvalidated or evicted (step 618), and the data at the load address maybe fetched into the cache line, the appropriate MESI bits 130 may beset, and the line is not marked as a read-set cache line of thetransaction, i.e., read-bit 132/248 is not set (step 622).

If all cache lines in the congruence class of the load address haveeither a read-bit or write-bit, 132/244 or 138/252, respectively, set(decision step 616, “N” branch), the load data may be read into one ormore general registers (step 620) and made available to the transaction.

If the load instruction is to be monitored, i.e., the cache line toreceive the data fetched from the load address will be included in aread-set cache line of the transaction (decision step 602, “Y” branch),the system determines if the load address is in a non-conflict RDT or RTaddress range (decision step 604), and thus does not need monitoring oris monitored by a monitoring scheme that does not depend on read-setmembership. If the load address is in an RDT or RT address range(decision step 604, “Y” branch) then the process of steps 614 through622, as described above, and as appropriate, may be performed, and thedata at the load address may be either fetched into an open cache linethat will not be marked as a read-set cache line of the transaction(step 622), or the data at the load address may be read into one or moregeneral registers (step 620) and made available to the transaction.

If the load instruction is to be monitored (decision step 602, “Y”branch) and the load address is not in an RDT or RT address range(decision step 604, “N” branch), then the system determines if any cachelines in the congruence class of the load address can be identified asnot being a read-set or a write-set cache line of a transaction(decision step 606). If all cache lines in the congruence class of theload address are read-set or a write set cache lines of a transaction(decision step 606, “N” branch), then the transaction may be aborted(step 608). In certain embodiments, a determination may be made as towhether another transaction can be aborted, thereby freeing up a line inthe congruence class of the load address, so that the presenttransaction can continue.

If a cache line in the congruence class of the load address is not aread-set or a write set cache line of a transaction (decision step 606,“Y” branch), the cache line may be invalidated or evicted (step 610),and the data at the load address may be fetched into the cache line, theappropriate MESI bits 130 may be set, and the line may be marked as aread-set cache line of the transaction, i.e., read-bit 132/248 is notset (step 612).

FIGS. 7A and 7B is a flowchart depicting operational steps forperforming a another aspect of cache line evictions during transactionalexecution of a process, in accordance with an embodiment of thedisclosure. A processor begins executing a TM transaction (step 700). Inan embodiment, the processor may receive the instructions to executefrom an issue queue (730). In the course of executing the TMinstruction, the processor begins executing a memory data accessinginstruction (step 702), received, for example, from the issue queue(730). The processor fetches the memory data referenced in the memorydata accessing instruction (step 704) from, for example, its processormemory (734), and loads the memory data into, for example, its processorhardware registers (732). The processor determines if the number ofcache lines in its private cache (736) that are marked as read-set cachelines of the transaction is greater than or equal to a defined thresholdvalue (decision step 706). If the number of cache lines in its privatecache (736) that are marked as read-set cache lines of the transactionis not greater than or equal to a defined threshold value (decision step706, “N” branch), then the processor may load the fetched memory datafrom its hardware registers (732) into a line of its private cache (736)(step 722) and may mark the line as a read-set cache line of thetransaction, and this processing ends.

If the number of cache lines in the processor's private cache (736) thatare marked as read-set cache lines of the transaction is greater than orequal to the defined threshold value (decision step 706, “Y” branch),then the processor determines if there is a read-set cache line of thetransaction in the congruence class of the memory address of the fetchedmemory data that contains data from a defined non-conflict address range(decision step 708). In an embodiment, the non-conflict address rangemay be defined in the parameters of a range-setting instruction (738)executed as part of the TM transaction, and the defined non-conflictaddress range may be stored in one or more non-conflict range IDhardware registers (740). If there is not a read-set cache line of thetransaction in the congruence class of the memory address of the fetchedmemory data that contains data from a defined non-conflict address range(decision step 708, “N” branch), then the TM transaction may be aborted(step 724) and this processing ends.

If there is a read-set cache line of the transaction in the congruenceclass of the memory address of the fetched memory data that containsdata from a defined non-conflict address range (decision step 708, “Y”branch), then the identified read-set cache line in the congruence classis invalidated in the cache (736) (step 710), and the fetched memorydata is, for example, loaded from the processor hardware registers (732)into the cache line (736), and the line is marked as a read-set cacheline of the transaction (step 712).

In an embodiment, in the course of executing the TM transaction, theprocessor may receive a cache coherency request over the processor'sinterconnect bus (742) from, for example, another processor coupled tothe bus (step 714). The processor may determine if the cache coherencyrequest conflicts with any addresses in the defined non-conflict addressrange(s) (decision step 716). In an embodiment, the processor may use acache coherency conflict detector (744), which may include software,firmware, and hardware components, and may be coupled to theinterconnect bus (742), the processor's private cache (736), and thenon-conflict range ID hardware registers (740) to aid in determining ifa cache coherency conflict exists.

If the cache coherency request does not conflict with any addresses inthe defined non-conflict address range(s) (decision step 716, “N”branch), then the processor processes the non-conflicting cachecoherency request (step 720), and this processing ends. If the cachecoherency request does conflict with any addresses in the definednon-conflict address range(s) (decision step 716, “Y” branch), then theprocessor aborts the TM transaction (step 718) and this processing ends.

FIG. 8 depicts a block diagram of components of an embodiment of acomputing device 800 that may include the multicore transactional memoryenvironment of FIG. 1, the CPU core of a multicore transactional memoryenvironment of FIG. 2, and/or the components of the CPU of FIG. 3, inaccordance with an embodiment of the disclosure. It should beappreciated that FIG. 8 provides only an illustration of oneimplementation and does not imply any limitations with regard to theenvironments in which different embodiments may be implemented. Manymodifications to the depicted environment may be made.

Computing device 800 can include one or more processors 802, one or morecomputer-readable RAMs 804, one or more computer-readable ROMs 806, oneor more computer readable storage media 808, device drivers 812,read/write drive or interface 814, and network adapter or interface 816,all interconnected over a communications fabric 818. Communicationsfabric 818 can be implemented with any architecture designed for passingdata and/or control information between processors (such asmicroprocessors, communications and network processors, etc.), systemmemory, peripheral devices, and any other hardware components within asystem.

One or more operating systems 810, and application programs 828 may bestored on one or more of the computer-readable tangible storage devices808 for execution by one or more of the processors 802 via one or moreof the RAMs 804 (which typically include cache memory). In theillustrated embodiment, each of the computer readable storage media 808can be a magnetic disk storage device of an internal hard drive, CD-ROM,DVD, memory stick, magnetic tape, magnetic disk, optical disk, asemiconductor storage device such as RAM, ROM, EPROM, flash memory orany other computer readable storage media that can store a computerprogram and digital information. Computing device 800 can also include aR/W drive or interface 814 to read from and write to one or moreportable computer readable storage media 826.

Computing device 800 can also include a network adapter or interface816, such as a TCP/IP adapter card or wireless communication adapter.Information, such as application program(s) 828, can be downloaded tocomputing device 800 from an external computer or external storagedevice via a network (for example, the Internet, a local area network orother, wide area network or wireless network) and network adapter orinterface 816. From the network adapter or interface 816, theinformation may loaded into computer readable storage media 808. Thenetwork may comprise copper wires, optical fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers.

Computing device 800 can also include a display screen 820, a keyboardor keypad 822, and a computer mouse or touchpad 824. Device drivers 812may interface to display screen 820 for imaging, to keyboard or keypad822, to computer mouse or touchpad 824, or to display screen 820 forpressure sensing of alphanumeric character entry and user selections.The device drivers 812, R/W drive or interface 814 and network adapteror interface 816 can comprise hardware and software (stored in computerreadable storage media 808 and/or ROM 806).

The present embodiments may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent embodiments.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present embodiments may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Java, Smalltalk, C++ or the like,and conventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present embodiments.

Aspects of the present embodiments are described herein with referenceto flowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the invention, and these are,therefore, considered to be within the scope of the invention, asdefined in the following claims.

What is claimed is:
 1. A computer program product for managing cacheline evictions during a transaction executing in a transactional memoryenvironment comprising: a computer operating system embodied on acomputer readable storage medium having program instructions executableby a processor, the program instructions comprising: executing, by theprocessor, a non-conflict address range setting instruction, wherein:the non-conflict address range setting instruction is aread-and-do-not-track instruction; wherein data fetched into a cacheline that contains data from the non-conflict address range is notmarked as a read-set cache line of the transaction; wherein the cacheline that contains data from the non-conflict range is protected byprotection facilities, the protection facilities including DynamicAddress Translation, access list control, and hardware key; and whereinstoring, by another processor, into a cache line containing at least aportion of the non-conflict address range, is not permitted; in responseto the processor executing a memory data accessing instruction during atransaction, determining, by a cache manager, whether a pre-definedmaximum number of read-set cache lines is reached; based on thepre-defined maximum number of read-set cache lines being reached,determining, by the cache manager, whether a memory address operand ofthe memory data accessing instruction is within the non-conflict addressrange; based on the pre-defined maximum number of read-set cache linesbeing reached, and based on a memory address operand of the memory dataaccessing instruction being within the non-conflict address range,evicting a read-set cache line having data from a memory address in thenon-conflict address range; and completing, by the processor, the memorydata accessing instruction.
 2. The computer program product of claim 1,wherein the non-conflict address range setting instruction comprises anyone of: a non-conflict address range setting instruction having aplurality of read-track registers, wherein each of the plurality ofread-track registers includes a starting non-conflict address and anending non-conflict address, and wherein the plurality of read-trackregisters are linked together and searchable as a linked list; anon-conflict address range setting instruction having the non-conflictaddress range as a parameter; an instruction initiating the transactionin the transactional memory environment, wherein thetransaction-initiating instruction includes the non-conflict addressrange as the parameter.
 3. The computer program product of claim 1,wherein the non-conflict address range is defined at a start of thetransaction and is in effect for the duration of the transaction.
 4. Thecomputer program product of claim 1, wherein the non-conflict addressrange comprises: memory addresses including constants, fixed data, andnon-modifiable data.
 5. The computer program product of claim 1, furthercomprising: aborting the transaction, based on 1) the maximum number ofread-set cache lines being reached; 2) no cache line being identified asa read-set cache line of a transaction; 3) no cache line containing anaddress within the non-conflict address range; and 4) no cache linebeing protected by the protection facilities.
 6. The computer programproduct of claim 1, further comprising: not aborting the transaction andstoring the memory address operand of the memory data accessinginstruction in one or more general purpose registers, based on 1) themaximum number of read-set cache lines being reached; 2) no cache linebeing identified as a read-set cache line of a transaction; 3) no cacheline containing an address within the non-conflict address range; and 4)no cache line being protected by protection facilities.
 7. A computersystem for managing cache line evictions during a transaction executingin a transactional memory environment, the computer system comprising: amemory; and a processor in communications with the memory, wherein thecomputer system is configured to perform a method, the methodcomprising: executing, by the processor, a non-conflict address rangesetting instruction, wherein: the non-conflict address range settinginstruction is a read-and-do-not-track instruction; wherein data fetchedinto a cache line that contains data from the non-conflict address rangeis not marked as a read-set cache line of the transaction; wherein thecache line that contains data from the non-conflict range is protectedby protection facilities, the protection facilities including DynamicAddress Translation, access list control, and hardware key; and whereinstoring, by another processor, into a cache line containing at least aportion of the non-conflict address range, is not permitted; in responseto the processor executing a memory data accessing instruction during atransaction, determining, by a cache manager, whether a pre-definedmaximum number of read-set cache lines is reached; based on thepre-defined maximum number of read-set cache lines being reached,determining, by the cache manager, whether a memory address operand ofthe memory data accessing instruction is within the non-conflict addressrange; based on the pre-defined maximum number of read-set cache linesbeing reached, and based on a memory address operand of the memory dataaccessing instruction being within the non-conflict address range,evicting a read-set cache line having data from a memory address in thenon-conflict address range; and completing, by the processor, the memorydata accessing instruction.
 8. The computer system of claim 7, whereinthe non-conflict address range setting instruction comprises any one of:a non-conflict address range setting instruction having a plurality ofread-track registers, wherein each of the plurality of read-trackregisters includes a starting non-conflict address and an endingnon-conflict address, and wherein the plurality of read-track registersare linked together and searchable as a linked list; a non-conflictaddress range setting instruction having the non-conflict address rangeas a parameter; an instruction initiating the transaction in thetransactional memory environment, wherein the transaction-initiatinginstruction includes the non-conflict address range as the parameter. 9.The computer system of claim 7, wherein the non-conflict address rangeis defined at a start of the transaction and is in effect for theduration of the transaction.
 10. The computer system of claim 7, whereinthe non-conflict address range comprises: memory addresses includingconstants, fixed data, and non-modifiable data.
 11. The computer systemof claim 7, further comprising: aborting the transaction, based on 1)the maximum number of read-set cache lines being reached; 2) no cacheline being identified as a read-set cache line of a transaction; 3) nocache line containing an address within the non-conflict address range;and 4) no cache line being protected by the protection facilities. 12.The computer system of claim 7, further comprising: not aborting thetransaction and storing the memory address operand of the memory dataaccessing instruction in one or more general purpose registers, basedon 1) the maximum number of read-set cache lines being reached; 2) nocache line being identified as a read-set cache line of a transaction;3) no cache line containing an address within the non-conflict addressrange; and 4) no cache line being protected by protection facilities.